lib: (pwdutils.c) improve error return protocol for callers

lib: (pwdutils.c) improve error return protocol for callers

In some cases libc does not properly set errno when no passwd
or group entry was found by getpwnam or getgrnam like functions.
This becomes an issue when callers have to provide coherent error
messages with err(). Therefore, let us improve the errno protocol
in internal wrapper functions for the above mentioned and set errno
 when libc does not.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>