vtls: add MesaLink to curl_sslbackend enum

vtls: add MesaLink to curl_sslbackend enum

MesaLink support was added in commit 57348eb97d1b8fc3742e02c 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>