* libmisc/chkname.h, libmisc/chkname.c: check_group_name (resp.

	* libmisc/chkname.h, libmisc/chkname.c: check_group_name (resp.
	check_user_name) renamed to is_valid_user_name (resp.
	is_valid_group_name). is_valid_user_name and is_valid_group_name
	return a bool.
	* src/grpck.c, src/newusers.c, src/usermod.c, src/useradd.c,
	src/groupmod.c, src/pwck.c, src/groupadd.c: Use is_valid_user_name
	and is_valid_group_name, following above change.
	* libmisc/chkname.c: Avoid implicit conversion of chars to
	booleans. Add brackets and parenthesis.