login: Use getpwnam_r() instead of getpwnam().

Since we expect the entry to still be valid after calling into PAM,
which may call getpwnam() itself, we need to use getpwnam_r().

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans, imp, allanjude, markj
Differential Revision: https://reviews.freebsd.org/D43376