pkexec: Support just plain "pkexec" to run shell

I use "pkexec bash" a lot, and it'd be nice to be able to just type
"pkexec" and have it do what I want.  Like how "su" will run the shell
by default.

https://bugs.freedesktop.org/show_bug.cgi?id=74933
build: Fix several issues on FreeBSD

 1. Fallback to fsync() if fdatasync() is not available.
 2. Check whether setnetgrent() has a return value.
 3. Check whether the system has SIGPOLL.
 4. Add configure option to disable test. mocklibc cannot be built on FreeBSD
    because of function prototype conflict.

https://bugs.freedesktop.org/show_bug.cgi?id=73821