time-util: change parse_sec_fix_0() to accept "0s" for infinity too (#10501)

This function is about compatibility, nothing else, hence we should make
it properly compatible.

Fixes: #9556

src/basic/time-util.c

@@ -1074,18 +1074,19 @@ int parse_sec(const char *t, usec_t *usec) {
         return parse_time(t, usec, USEC_PER_SEC);
 }
 
-int parse_sec_fix_0(const char *t, usec_t *usec) {
+int parse_sec_fix_0(const char *t, usec_t *ret) {
+        usec_t k;
+        int r;
+
         assert(t);
-        assert(usec);
+        assert(ret);
 
-        t += strspn(t, WHITESPACE);
+        r = parse_sec(t, &k);
+        if (r < 0)
+                return r;
 
-        if (streq(t, "0")) {
-                *usec = USEC_INFINITY;
-                return 0;
-        }
-
-        return parse_sec(t, usec);
+        *ret = k == 0 ? USEC_INFINITY : k;
+        return r;
 }
 
 int parse_nsec(const char *t, nsec_t *nsec) {