runuser: new command (derived from su(1))

runuser: new command (derived from su(1))

 This command is based on su(1), the differences:

 - based on Fedora runuser su(1) patch
 - not installed with suid rights
 - allowed for root users only
 - don't ask for password
 - uses PAM session, for example:

$ cat /etc/pam.d/runuser
auth		sufficient	pam_rootok.so
session		optional	pam_keyinit.so revoke
session		required	pam_limits.so
session		required	pam_unix.so

$ cat /etc/pam.d/runuser-l
auth		include		runuser
session		optional	pam_keyinit.so force revoke
session		include		runuser

Signed-off-by: Karel Zak <kzak@redhat.com>