Parse "timeout=0" as infinity in various generators (#6264)

This extends 2d79a0bbb9 to the kernel
command line parsing.

The parsing is changed a bit to only understand "0" as infinity. If units are
specified, parse normally, e.g. "0s" is just 0. This makes it possible to
provide a zero timeout if necessary.

Simple test is added.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1462378.

src/basic/time-util.c

@@ -1009,6 +1009,16 @@ 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) {
+        t += strspn(t, WHITESPACE);
+        if (streq(t, "0")) {
+                *usec = USEC_INFINITY;
+                return 0;
+        }
+
+        return parse_sec(t, usec);
+}
+
 int parse_nsec(const char *t, nsec_t *nsec) {
         static const struct {
                 const char *suffix;