For congruency sake with the naming of other CURL_XXXXXX_CURL_OFF_T macros,

the names of the curl_off_t formatting string directives now become
CURL_FORMAT_CURL_OFF_T and CURL_FORMAT_CURL_OFF_TU.

CURL_FMT_OFF_T  -> CURL_FORMAT_CURL_OFF_T
CURL_FMT_OFF_TU -> CURL_FORMAT_CURL_OFF_TU

Remove the use of an internal name for the curl_off_t formatting string directives
and use the common one available from the inside and outside of the library.

FORMAT_OFF_T  -> CURL_FORMAT_CURL_OFF_T
FORMAT_OFF_TU -> CURL_FORMAT_CURL_OFF_TU