udevd: use ppoll instead of signal pipes

udevd uses a rather old-fashioned way of handling signals
while waiting for input through select (ie by using an unnamed
pipe, to which the signal handler writes one byte for every signal
received). This is rather awkward and may potentially even block
if we receive more signals than the kernel's pipe buffer.

This patch replaces all of that with ppoll, which was designed
for this purpose.

It also removes the SA_RESTART flag from all installed signal
handlers, because otherwise the ppoll call would just be restarted
after handling eg a SIGCHLD.

Signed-off-by: Olaf Kirch <okir@suse.de>