selinux: implement missing selinux functions

checkPasswdAccess() is deprecated
http://linux.die.net/man/3/selinux_check_passwd_access

also, ensure context is initialized in run_shell()

Note: these stubs are just compatible with busybox source code
Should be enhanced, or implemented in libselinux...

Change-Id: I22ea2679191d5c0887b6ef8c77f09a032c226876