common: New function string_to_u64.
* common/stringhelp.c (string_to_u64): New. * dirmngr/http.c (longcounter_t): Remove. (struct cookie_s): Change content_length to uint64_t. (parse_response): Use string_to_u64.
Meanwhile we allow some C99 features including stdint.h. Thus we can
simplify things now.
- Signed-off-by: Werner Koch <wk@gnupg.org>