curl.h: change some enums to defines with L suffix

curl.h: change some enums to defines with L suffix

To help applications do the right thing easier, change some enum values
into defines with L suffixes so that they get the corect type (long)
easier when used with curl_easy_setopt(). This also fixes a few of our
own libtests.

To reduce the risk that this change breaks the compile for any existing
users, the previously provided enums are still provided, but the values
to use are not defined by the enums.

This change "magically" fixes a few RTSP test failures we have had on
64-bit platforms because those options were not see using longs
properly.

Closes #16482