Introduce three additional MAC Framework and MAC Policy entry points to
control socket poll() (select()), fstat(), and accept() operations,
required for some policies:

poll()          mac_check_socket_poll()
fstat()         mac_check_socket_stat()
accept()        mac_check_socket_accept()

Update mac_stub and mac_test policies to be aware of these entry points.
While here, add missing entry point implementations for:

mac_stub.c      stub_check_socket_receive()
mac_stub.c      stub_check_socket_send()
mac_test.c      mac_test_check_socket_send()
mac_test.c      mac_test_check_socket_visible()

Obtained from: TrustedBSD Project
Sponsored by: SPAWAR, SPARTA