Prefix enum constants in OCSP callback

The enum values introduced in 8d3aa32 for CDRIVER-3408 appear to conflict with existing constants on Windows. I picked this up during an Appveyor build for the PHP driver (see: here).

I suggest changing these to more unique names. Additionally, there are a few cases within _mongoc_ocsp_tlsext_status_cb where the SUCCESS case in our enum is used to check return values from other TLS functions (e.g. OCSP_basic_verify, X509_check_host). I'd suggest we define explicit constants for those functions rather than re-use our internal enum for the callback's return value.