MesaLink support was added in commit 57348eb97d but the
backend was never added to the curl_sslbackend enum in curl/curl.h.
This adds the new backend to the enum and updates the relevant docs.
Closes#3195
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
it is a one time *set*, not necessarily a one time use... it can be
called again if the first call failed or just listed the alternatives.
clarify that the available backends are the ones this build supports
plus add some formatting
Reported-by: Rich Gray
Bug: https://curl.haxx.se/mail/lib-2017-08/0119.html
