vtls: select ssl backend case-insensitive (follow-up)

- Do a case-insensitive comparison of CURL_SSL_BACKEND env as well.

- Change Curl_strcasecompare calls to strcasecompare
  (maps to the former but shorter).

Follow-up to c290b8fb.

Bug: https://github.com/curl/curl/commit/c290b8f#commitcomment-24094313

Co-authored-by: Jay Satiro