+ Use _PATH_GROUP and _PATH_MASTERPASSWD (from OpenBSD)
+ Use -G group1,group2,group3 for multiple groups in useradd and usermod
(pointed out by Matt Green, and also changed in OpenBSD, but done more
efficiently here)
+ is_number should not be inside #ifdef EXTENSIONS (from OpenBSD)
+ clear up yet another usage message (for user(8) and group(8)) - noticed
in passing, unknown if fixed anywhere else
468671f5be