flua: fbsd: return a process handle to operate on when we exec()

This gives us some way to be able to write to stdin if we want to, or
as a future improvement, will allow us to extract stdout from the
process. The handle is setup to close and waitpid() on close/gc so that
existing users wouldn't necessarily leak for the lifetime of the script
if they weren't adopted to the new model.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D50538