everywhere: port everything to sigprocmask_many() and friends

This ports a lot of manual code over to sigprocmask_many() and friends.

Also, we now consistly check for sigprocmask() failures with
assert_se(), since the call cannot realistically fail unless there's a
programming error.

Also encloses a few sd_event_add_signal() calls with (void) when we
ignore the return values for it knowingly.

src/basic/signal-util.c

@@ -159,7 +159,7 @@ int sigset_add_many(sigset_t *ss, ...) {
         return r;
 }
 
-int sigprocmask_many(int how, ...) {
+int sigprocmask_many(int how, sigset_t *old, ...) {
         va_list ap;
         sigset_t ss;
         int r;
@@ -174,7 +174,7 @@ int sigprocmask_many(int how, ...) {
         if (r < 0)
                 return r;
 
-        if (sigprocmask(how, &ss, NULL) < 0)
+        if (sigprocmask(how, &ss, old) < 0)
                 return -errno;
 
         return 0;

src/basic/signal-util.h

@@ -33,7 +33,7 @@ int default_signals(int sig, ...);
 int sigaction_many(const struct sigaction *sa, ...);
 
 int sigset_add_many(sigset_t *ss, ...);
-int sigprocmask_many(int how, ...);
+int sigprocmask_many(int how, sigset_t *old, ...);
 
 const char *signal_to_string(int i) _const_;
 int signal_from_string(const char *s) _pure_;