useradd: fix OpenBSD support

OpenBSD useradd(8) command sligtly differs from Linux's.
/etc/login.defs is used instead of /etc/login.defs, some options are
unsupported and some usermod(8) switches are different.
Revert "useradd: fix OpenBSD support"

This reverts commit 85c758ecf1b3e6e93a9e37c4152621b069b9c4f2.