unshare: Add some helpers for forking and synchronizing

unshare: Add some helpers for forking and synchronizing

There is (or rather, will be) a common pattern in unshare like

	/* parent */	/* child */
	fork()
	do_some_work()
	sync()		wait();
			do_more_work();
	wait()		exit();

where the parent has to do some tasks (unshare(), fork() again, etc)
before the child can do its work. At the moment this is implemented
explicitly with a pipe().

Add some helper functions to abstract this process away. In addition,
switch to eventfd() instead of pipe(). As the man page for eventfd(2)
notes,

> Applications can use an eventfd file descriptor instead of a pipe (see
> pipe(2)) in all cases where a pipe is used simply to signal events. The
> kernel overhead of an eventfd file descriptor is much lower than that of
> a pipe, and only one file descriptor is required (versus the two required
> for a pipe).

Signed-off-by: Sean Anderson <seanga2@gmail.com>