Add a placeholder for the execve_mac() system call, similar to SELinux's

Add a placeholder for the execve_mac() system call, similar to SELinux's
execve_secure() system call, which permits a process to pass in a label
for a label change during exec.  This permits SELinux to change the
label for the resulting exec without a race following a manual label
change on the process.  Because this interface uses our general purpose
MAC label abstraction, we call it execve_mac(), and wrap our port of
SELinux's execve_secure() around it with appropriate sid mappings.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories