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/timesync/timesyncd-manager.c

@@ -1132,15 +1132,21 @@ 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_add_signal(m->event, NULL, SIGRTMIN+18, sigrtmin18_handler, NULL);
+        r = sd_event_set_signal_exit(m->event, true);
+        if (r < 0)
+                return r;
+
+        r = sd_event_add_signal(m->event, /* ret_event_source= */ NULL, (SIGRTMIN+18)|SD_EVENT_SIGNAL_PROCMASK, sigrtmin18_handler, /* userdata= */ NULL);
+        if (r < 0)
+                log_debug_errno(r, "Failed to install SIGRTMIN+18 signal handler, ignoring: %m");
 
         r = sd_event_add_memory_pressure(m->event, NULL, NULL, NULL);
         if (r < 0)
                 log_debug_errno(r, "Failed allocate memory pressure event source, ignoring: %m");
 
-        (void) sd_event_set_watchdog(m->event, true);
+        r = sd_event_set_watchdog(m->event, true);
+        if (r < 0)
+                log_debug_errno(r, "Failed to enable watchdog handling, ignoring: %m");
 
         /* Load previous synchronization state */
         r = access("/run/systemd/timesync/synchronized", F_OK);

src/timesync/timesyncd.c

@@ -179,8 +179,6 @@ 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, SIGRTMIN+18) >= 0);
-
         r = manager_new(&m);
         if (r < 0)
                 return log_error_errno(r, "Failed to allocate manager: %m");

src/timesync/wait-sync.c

@@ -184,19 +184,13 @@ static int run(int argc, char * argv[]) {
         };
         int r;
 
-        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT) >= 0);
-
         r = sd_event_default(&event);
         if (r < 0)
                 return log_error_errno(r, "Failed to allocate event loop: %m");
 
-        r = sd_event_add_signal(event, NULL, SIGTERM, NULL, NULL);
+        r = sd_event_set_signal_exit(event, true);
         if (r < 0)
-                return log_error_errno(r, "Failed to create sigterm event source: %m");
-
-        r = sd_event_add_signal(event, NULL, SIGINT, NULL, NULL);
-        if (r < 0)
-                return log_error_errno(r, "Failed to create sigint event source: %m");
+                return log_error_errno(r, "Failed to enable SIGTERM/SIGINT handling: %m");
 
         r = sd_event_set_watchdog(event, true);
         if (r < 0)