mkdir,mkfifo,mknod: with -Z, create SMACK security context

mkdir,mkfifo,mknod: with -Z, create SMACK security context

Enable creation of SMACK security context with -Z command-line switch
if SMACK is enabled.

* mkdir.c (main): Set process security context to given SMACK label.
* mkfifo.c (main): Likewise.
* mknod.c (main): Likewise.
* src/local.mk: link mk{dir, fifo, nod} with libsmack.
* NEWS: Mention the new feature.