tools/nolibc/unistd: extract msleep(), sleep(), tcsetpgrp() to unistd.h

These functions are normally provided by unistd.h. For ease of porting,
let's create the file and move them there.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>