mutt_str_atol: better error handling

mutt_str_atol: better error handling

Reset errno to 0 before calling strtol and testing it in mutt_atol.

Otherwise providing LONG_MAX+1 then LONG_MAX gave an error for LONG_MAX too.

* Detect overflow on negative numbers too (< LONG_MIN).

* Distinguish between format error and number overflow as already
  expected (e.g. for pgp_timeout and smime_timeout).