tree-wide: use assert_se() for signal operations with constants

Continuation of a3ebe5eb62:
in other places we sometimes use assert_se(), and sometimes normal error
handling. sigfillset and sigaddset can only fail if mask is NULL (which cannot
happen if we are passing in a reference), or if the signal number is invalid
(which really shouldn't happen when we are using a constant like SIGCHLD. If
SIGCHLD is invalid, we have a bigger problem). So let's simplify things and
always use assert_se() in those cases.

In sigset_add_many() we could conceivably pass an invalid signal, so let's keep
normal error handling here. The caller can do assert_se() around the
sigprocmask_many() call if appropriate.

'>= 0' is used for consistency with the rest of the codebase.

src/basic/async.c

@@ -32,10 +32,7 @@ int asynchronous_job(void* (*func)(void *p), void *arg) {
                 goto finish;
         }
 
-        if (sigfillset(&ss) < 0) {
-                r = -errno;
-                goto finish;
-        }
+        assert_se(sigfillset(&ss) >= 0);
 
         /* Block all signals before forking off the thread, so that the new thread is started with all signals
          * blocked. This way the existence of the new thread won't affect signal handling in other threads. */