executor: really set POSIX_SPAWN_SETSIGDEF for posix_spawn

posix_spawnattr_setflags() doesn't OR the input to the current set of flags,
it overwrites them, so we are currently losing POSIX_SPAWN_SETSIGDEF.

Follow-up for: 6ecdfe7

src/basic/process-util.c

@@ -2054,10 +2054,8 @@ int posix_spawn_wrapper(const char *path, char *const *argv, char *const *envp,
         r = posix_spawnattr_init(&attr);
         if (r != 0)
                 return -r; /* These functions return a positive errno on failure */
-        r = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK);
-        if (r != 0)
-                goto fail;
-        r = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGDEF); /* Set all signals to SIG_DFL */
+        /* Set all signals to SIG_DFL */
+        r = posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSIGMASK|POSIX_SPAWN_SETSIGDEF);
         if (r != 0)
                 goto fail;
         r = posix_spawnattr_setsigmask(&attr, &mask);