... to cater for systems with unsigned time_t variables.
- Renamed the functions to curlx_timediff and Curl_timediff_us.
- Added overflow protection for both of them in either direction for
both 32 bit and 64 bit time_ts
- Reprefixed the curlx_time functions to use Curl_*
Reported-by: Peter Piekarski
Fixes#2004Closes#2005