flua: lposix: add more useful functions for general purpose scripts

unistd:

• _exit
• close
• fork
• getpid
• pipe
• read
• write

libgen:

• basename, dirname

stdlib:

• realpath

These are sufficient for a number of real world scenarios. In our first
application of them, we use the libgen+stdlib additions to grab the
script dir based on argv[0]. The unistd assortment is then used to
outsource a bunch of work to forks and report back to the main process.

Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D39083