tree-wide: add new SIGNAL_VALID() macro-like function that validates signal numbers

And port all code over to use it.

src/basic/signal-util.c

@@ -255,7 +255,7 @@ int signal_from_string(const char *s) {
         }
         if (safe_atou(s, &u) >= 0) {
                 signo = (int) u + offset;
-                if (signo > 0 && signo < _NSIG)
+                if (SIGNAL_VALID(signo))
                         return signo;
         }
         return -EINVAL;

src/basic/signal-util.h

@@ -50,3 +50,7 @@ static inline void block_signals_reset(sigset_t *ss) {
                 assert_se(sigprocmask_many(SIG_BLOCK, &t, __VA_ARGS__, -1) >= 0); \
                 t;                                                                \
         })
+
+static inline bool SIGNAL_VALID(int signo) {
+        return signo > 0 && signo < _NSIG;
+}