lposix: Use reentrant passwd and group lookup functions

The implementation of chown() in the posix module handles user and group
names as well as numeric IDs. When resolving names, be sure to use
reentrant lookup functions rather than assuming it's safe to clobber the
internal buffers used by getpwnam() and getgrnam().

Fix some style nits while here.

Reviewed by: imp, bapt
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46555