api: implement user notification in libseccomp

api: implement user notification in libseccomp

This patch is heavily based on an earlier patchset by Tycho
Andersen.  I took Tycho's patch and incorporated the requested changes
from the review, fixed some corner case bugs, and simplified the API
a bit.

Kernel 5.0 includes the new user notification return code. Here's all the
infrastructure to handle that.

The idea behind the user notification return code is that the filter stops
the syscall, and forwards it to a "listener fd" that is created when
installing a filter. Then then some userspace task can listen and process
events accordingly by taking some (or no) action in userspace, and then
returning a value from the command.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: Paul Moore <paul@paul-moore.com>