sshd: don't use argv[0] as PAM service name

sshd would implicitly use argv[0] as the PAM service name to
allow people to select different PAM service names by making
differently-named copies/links to the sshd binary.

Splitting sshd into sshd/sshd-session broke this, as the process
that starts PAM is always sshd-session and the user has no control
over this.

Hardcode "sshd" as the default PAM service name unless/until we
figure out a better way. Should unbreak OSX integration tests.

auth-pam.c

@@ -68,8 +68,7 @@
 #endif
 
 #if !defined(SSHD_PAM_SERVICE)
-extern char *__progname;
-# define SSHD_PAM_SERVICE __progname
+# define SSHD_PAM_SERVICE "sshd"
 #endif
 
 /* OpenGroup RFC86.0 and XSSO specify no "const" on arguments */