lib/, src/, po/: pw_entry(): Move function to src/sulogin.c

lib/, src/, po/: pw_entry(): Move function to src/sulogin.c

That's the only file where it's called, and it's a delicate function.
Reduce the chances that other files call it.

Link: <https://github.com/shadow-maint/shadow/pull/908>
Suggested-by: Samanta Navarro <ferivoz@riseup.net>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>