Add licensing and copyright information for all files in this repository. This
either happens in the file itself as a comment header or in the file
`.reuse/dep5`.
This commit also adds a Github workflow to check pull requests and adapts
copyright.pl to the changes.
Closes#8869
... 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
