Switch flag definitions to be on pflag instead of flag in package go/vt/servenv

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/servenv/*.go
go/vt/servenv/buildinfo.go:     Version = flag.Bool("version", false, "print binary version")
go/vt/servenv/grpc_server.go:   GRPCPort = flag.Int("grpc_port", 0, "Port to listen on for gRPC calls")
go/vt/servenv/grpc_server.go:   GRPCCert = flag.String("grpc_cert", "", "server certificate to use for gRPC connections, requires grpc_key, enables TLS")
go/vt/servenv/grpc_server.go:   GRPCKey = flag.String("grpc_key", "", "server private key to use for gRPC connections, requires grpc_cert, enables TLS")
go/vt/servenv/grpc_server.go:   GRPCCA = flag.String("grpc_ca", "", "server CA to use for gRPC connections, requires TLS, and enforces client certificate check")
go/vt/servenv/grpc_server.go:   GRPCCRL = flag.String("grpc_crl", "", "path to a certificate revocation list in PEM format, client certificates will be further verified against this file during TLS handshake")
go/vt/servenv/grpc_server.go:   GRPCEnableOptionalTLS = flag.Bool("grpc_enable_optional_tls", false, "enable optional TLS mode when a server accepts both TLS and plain-text connections on the same port")
go/vt/servenv/grpc_server.go:   GRPCServerCA = flag.String("grpc_server_ca", "", "path to server CA in PEM format, which will be combine with server cert, return full certificate chain to clients")
go/vt/servenv/grpc_server.go:   GRPCAuth = flag.String("grpc_auth_mode", "", "Which auth plugin implementation to use (eg: static)")
go/vt/servenv/grpc_server.go:   GRPCMaxConnectionAge = flag.Duration("grpc_max_connection_age", time.Duration(math.MaxInt64), "Maximum age of a client connection before GoAway is sent.")
go/vt/servenv/grpc_server.go:   GRPCMaxConnectionAgeGrace = flag.Duration("grpc_max_connection_age_grace", time.Duration(math.MaxInt64), "Additional grace period after grpc_max_connection_age, after which connections are forcibly closed.")
go/vt/servenv/grpc_server.go:   GRPCInitialConnWindowSize = flag.Int("grpc_server_initial_conn_window_size", 0, "gRPC server initial connection window size")
go/vt/servenv/grpc_server.go:   GRPCInitialWindowSize = flag.Int("grpc_server_initial_window_size", 0, "gRPC server initial window size")
go/vt/servenv/grpc_server.go:   GRPCKeepAliveEnforcementPolicyMinTime = flag.Duration("grpc_server_keepalive_enforcement_policy_min_time", 10*time.Second, "gRPC server minimum keepalive time")
go/vt/servenv/grpc_server.go:   GRPCKeepAliveEnforcementPolicyPermitWithoutStream = flag.Bool("grpc_server_keepalive_enforcement_policy_permit_without_stream", false, "gRPC server permit client keepalive pings even when there are no active streams (RPCs)")
go/vt/servenv/grpc_server_auth_mtls.go: ClientCertSubstrings = flag.String("grpc_auth_mtls_allowed_substrings", "", "List of substrings of at least one of the client certificate names (separated by colon).")
go/vt/servenv/grpc_server_auth_static.go:       credsFile = flag.String("grpc_auth_static_password_file", "", "JSON File to read the users/passwords from.")
go/vt/servenv/mysql.go:var MySQLServerVersion = flag.String("mysql_server_version", "", "MySQL server version to advertise.")
go/vt/servenv/pid_file.go:var pidFile = flag.String("pid_file", "", "If set, the process will write its pid to the named file, and delete it on graceful shutdown.")
go/vt/servenv/pprof.go: _         = flag.String("cpu_profile", "", "deprecated: use '-pprof=cpu' instead")
go/vt/servenv/pprof.go: pprofFlag = flag.String("pprof", "", "enable profiling")
go/vt/servenv/pprof_test.go:    flag.Set("pprof", "cpu,waitSig")
go/vt/servenv/pprof_test.go:    flag.Set("pprof", "cpu")
go/vt/servenv/servenv.go:// After a server program has called flag.Parse, it needs to call
go/vt/servenv/servenv.go:       lameduckPeriod = flag.Duration("lameduck-period", 50*time.Millisecond, "keep running at least this long after SIGTERM before stopping")
go/vt/servenv/servenv.go:       onTermTimeout  = flag.Duration("onterm_timeout", 10*time.Second, "wait no more than this for OnTermSync handlers before stopping")
go/vt/servenv/servenv.go:       onCloseTimeout = flag.Duration("onclose_timeout", time.Nanosecond, "wait no more than this for OnClose handlers before stopping")
go/vt/servenv/servenv.go:       _              = flag.Int("mem-profile-rate", 512*1024, "deprecated: use '-pprof=mem' instead")
go/vt/servenv/servenv.go:       _              = flag.Int("mutex-profile-fraction", 0, "deprecated: use '-pprof=mutex' instead")
go/vt/servenv/servenv.go:       catchSigpipe   = flag.Bool("catch-sigpipe", false, "catch and ignore SIGPIPE on stdout and stderr if specified")
go/vt/servenv/servenv.go:       Port = flag.Int("port", 0, "port for the server")
go/vt/servenv/servenv.go:               flag.Usage()
go/vt/servenv/service_map.go:   flag.Var(&serviceMapFlag, "service_map", "comma separated list of services to enable (or disable if prefixed with '-') Example: grpc-queryservice")
go/vt/servenv/unix_socket.go:   SocketFile = flag.String("socket_file", "", "Local unix socket file to listen on")