pwd: implement POSIX pwd APIs.

Closes #150.
(getpwuid, setpwent): Specify correct behaviours.
* ext/posix/pwd.c (endpwent, getpwent, getpwnam, getpwuid)
(setpwent): Implement bindings to these POSIX pwd APIs.
* ext/posix/pwd.c (getpasswd): Remove.
* lib/posix.lua.in (getpasswd): Reimplement over POSIX APIs to
continue satisfying specifications.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>