tree-wide: make sigprocmask() changes more automatic

This tries to get rid of most manual sigprocmask() changes, in favour
of:

1. The SD_EVENT_SIGNAL_PROCMASK flag to sd_event_add_signal()
2. The sd_event_set_signal_exit() call for handling SIGTERM/SIGINT
3. Move masking of SIGWINCH into ptyfwd, out of nspawn/vmspawn/run

And while we are at it get rid of a bunch of event source fields whose
lifetime is bound to the sd_event object they belong to anyway, and make
use of the "floating" event source feature of sd-event instead.

src/resolve/resolved.c

@@ -67,8 +67,6 @@ static int run(int argc, char *argv[]) {
                         return log_error_errno(r, "Failed to drop privileges: %m");
         }
 
-        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT, SIGUSR1, SIGUSR2, SIGRTMIN+1, SIGRTMIN+18) >= 0);
-
         r = manager_new(&m);
         if (r < 0)
                 return log_error_errno(r, "Could not create manager: %m");