Track created curl_slist structs by option so they can be updated in situ.

At present, when curl_setopt() is called with an option that requires the
creation of a curl_slist, we simply push the new curl_slist onto a list to be
freed when the curl handle is freed. This avoids a memory leak, but means that
repeated calls to curl_setopt() on the same handle with the same option wastes
previously allocated memory on curl_slist structs that will no longer be read.

This commit changes the zend_llist that was previously used to track the lists
to a HashTable keyed by the option number, which means that we can simply
update the hash table each time curl_setopt() is called.

Fixes bug #65458 (curl memory leak).