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/string-util.c

@@ -1201,3 +1201,19 @@ size_t strspn_from_end(const char *str, const char *accept) {
 
         return n;
 }
+
+char *strdupspn(const char *a, const char *accept) {
+        if (isempty(a) || isempty(accept))
+                return strdup("");
+
+        return strndup(a, strspn(a, accept));
+}
+
+char *strdupcspn(const char *a, const char *reject) {
+        if (isempty(a))
+                return strdup("");
+        if (isempty(reject))
+                return strdup(a);
+
+        return strndup(a, strcspn(a, reject));
+}