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/oom/oomd-manager.c

@@ -657,11 +657,7 @@ int manager_new(Manager **ret) {
 
         (void) sd_event_set_watchdog(m->event, true);
 
-        r = sd_event_add_signal(m->event, NULL, SIGINT, NULL, NULL);
-        if (r < 0)
-                return r;
-
-        r = sd_event_add_signal(m->event, NULL, SIGTERM, NULL, NULL);
+        r = sd_event_set_signal_exit(m->event, true);
         if (r < 0)
                 return r;
 

src/oom/oomd.c

@@ -167,8 +167,6 @@ static int run(int argc, char *argv[]) {
         if (!FLAGS_SET(mask, CGROUP_MASK_MEMORY))
                 return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "Requires the cgroup memory controller.");
 
-        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT) >= 0);
-
         if (arg_mem_pressure_usec > 0 && arg_mem_pressure_usec < 1 * USEC_PER_SEC)
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "DefaultMemoryPressureDurationSec= must be 0 or at least 1s");