Importing usr.bin/msgc

Minix-specific changes were needed in msg_sys.def:

 * use minix_mmap()/minix_munmap() since mmap()/munmap() aren't
   defined on Minix.

 * use MAP_PRIVATE flag since MAP_SHARED isn't defined on Minix.

Tested with usr.bin/menuc/testm.

Change-Id: I1c463e7b0623036050adb6a9079c6747e1c1e084