vtls: convert the have_curlssl_* constants to runtime flags

vtls: convert the have_curlssl_* constants to runtime flags

The entire idea of introducing the Curl_ssl struct to describe SSL
backends is to prepare for choosing the SSL backend at runtime.

To that end, convert all the #ifdef have_curlssl_* style conditionals
to use bit flags instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>