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.h

@@ -240,3 +240,6 @@ bool streq_skip_trailing_chars(const char *s1, const char *s2, const char *ok);
 char *string_replace_char(char *str, char old_char, char new_char);
 
 size_t strspn_from_end(const char *str, const char *accept);
+
+char *strdupspn(const char *a, const char *accept);
+char *strdupcspn(const char *a, const char *reject);