core: clean-up signal reset logic when reexec

There's no need to save the old sigmask, if we are going to die. Let's
simplify this. Also, reset all the signal handlers, so that we don't
leave SIG_IGN set for some of them across reexec.

(cherry picked from commit 9bfcda9528)
(cherry picked from commit 92254f72bb)

src/core/main.c

@@ -1748,7 +1748,7 @@ finish:
         if (reexecute) {
                 const char **args;
                 unsigned i, args_size;
-                sigset_t ss, o_ss;
+                sigset_t ss;
 
                 /* Close and disarm the watchdog, so that the new
                  * instance can reinitialize it, but doesn't get
@@ -1834,8 +1834,10 @@ finish:
 
                 /* reenable any blocked signals, especially important
                  * if we switch from initial ramdisk to init=... */
-                sigemptyset(&ss);
-                sigprocmask(SIG_SETMASK, &ss, &o_ss);
+                reset_all_signal_handlers();
+
+                assert_se(sigemptyset(&ss) == 0);
+                assert_se(sigprocmask(SIG_SETMASK, &ss, NULL) == 0);
 
                 if (switch_root_init) {
                         args[0] = switch_root_init;
@@ -1855,8 +1857,6 @@ finish:
                         log_error("Failed to execute /bin/sh, giving up: %m");
                 } else
                         log_warning("Failed to execute /sbin/init, giving up: %m");
-
-                sigprocmask(SIG_SETMASK, &o_ss, NULL);
         }
 
         if (serialization)