Provide an implementation of mac_syscall() so that security modules

Provide an implementation of mac_syscall() so that security modules
can offer new services without reserving system call numbers, or
augmented versions of existing services.  User code requests a
target policy by name, and specifies the policy-specific API plus
target.  This is required in particular for our port of SELinux/FLASK
to the MAC framework since it offers additional security services.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, NAI Labs