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/network/wait-online/manager.c

@@ -410,8 +410,7 @@ int manager_new(Manager **ret,
         if (r < 0)
                 return r;
 
-        (void) sd_event_add_signal(m->event, NULL, SIGTERM, NULL, NULL);
-        (void) sd_event_add_signal(m->event, NULL, SIGINT, NULL, NULL);
+        (void) sd_event_set_signal_exit(m->event, true);
 
         if (timeout > 0) {
                 r = sd_event_add_time_relative(m->event, NULL, CLOCK_BOOTTIME, timeout, 0, NULL, INT_TO_PTR(-ETIMEDOUT));

src/network/wait-online/wait-online.c

@@ -204,8 +204,6 @@ static int run(int argc, char *argv[]) {
         if (arg_quiet)
                 log_set_max_level(LOG_ERR);
 
-        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT) >= 0);
-
         r = manager_new(&m, arg_interfaces, arg_ignore, arg_required_operstate, arg_required_family, arg_any, arg_timeout);
         if (r < 0)
                 return log_error_errno(r, "Could not create manager: %m");