Add support for CURLOPT_TCP_KEEPCNT (available since curl 8.9.0)

[dorrogeray]

Description

This options allows you to configure maximum number of keepalive probes:
https://curl.se/libcurl/c/CURLOPT_TCP_KEEPCNT.html
https://github.com/curl/curl/blob/767d5811b5c783b42cea999dd42ecf0453085d17/include/curl/curl.h#L2225

Normally, default value is used depending on OS configuration, but in some situation, you want to have control over this property, ie when communicating with a specific service.

[cmb69]

Yeah, makes sense to add this constant, too. Any volunteers?

Also note that you may want to run php ext/curl/sync-constants.php, to see what else is available in libcurl, but not in PHP source. And note that there may be open PRs regarding such constants.