Busybox "mount" applet unsupport "user" option and
result: must have _BB_SUID_NEVER applet option (my bug).
Last patch have reducing 216 bytes for "su" applet also.
- Made a new dir loginutils
- Moved all applets from pwd_grp to loginutils
- Added new applets su.c login.c to loginutils
- Made a new dir libpwdgrp
- Moved everything from pwd_grp/libpwd_grp there
- Added shadow.c to libpwdgrp
- Removed dir pwd_grp
- Added usage strings for login and su to usage.h
- Changed main Makefile to reflect the dir rearrangements
[Parts of this patch may overlap with my other two patches]