Migrate from Process (using bash -c) to posix_spawn

[Samasaur1]

I know the title makes it seem like we're going backwards in time, not forwards, but in my experience something like the following works better:

var pid: pid_t = 0
let args = ["/usr/bin/env"] + effectiveEditor + [dir + "/" + helpme + ".txt"]
let c_args = args.map { $0.withCString(strdup)! }
posix_spawn(&pid, c_args[0], nil, nil, c_args + [nil], environ)
waitpid(pid, nil, 0)

(from helpme-swiftly)

This should also work better with lots of arguments, especially quoted ones, since we're invoking the actual command rather than /bin/bash -c "command and arguments"