[RFC][PATCHSET] wait4()/waitid() cleanups

[Al Viro]

	This series switches compat wait4() and waitid() variants away
from set_fs() use.  A couple of helpers (kernel_wait4() and kernel_waitid())
are introduced, copying siginfo/rusage to userland is taken to sys_wait...().
That allows both to simplify the guts of do_wait() machinery and avoid
messing with set_fs() in compat variants of these syscalls.

	Lightly tested, seems to survive without regressions.  Please,
review.