privs: move drop_privileges out of xutils

Again, also to be able to maintain this more easily.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
proc: move out process specific management functions

Move them out of xutils, so that we can maintain them separately.
Also simplify things a bit.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>