string-util: add new strdupcspn()/strdupspn()

These combine strndup() + strspn()/strcspn() into one.

There are a bunch of strndupa() calls that could use similar treatment
(or should be converted to strdup[c]spn(), but this commit doesn't
bother with that.

src/basic/hostname-util.c

@@ -62,7 +62,7 @@ int gethostname_full(GetHostnameFlags flags, char **ret) {
         }
 
         if (FLAGS_SET(flags, GET_HOSTNAME_SHORT))
-                buf = strndup(s, strcspn(s, "."));
+                buf = strdupcspn(s, ".");
         else
                 buf = strdup(s);
         if (!buf)