protocols: use CURLPROTO_ internally

The PROT_* set of internal defines for the protocols is no longer
used. We now use the same bits internally as we have defined in the
public header using the CURLPROTO_ prefix. This is for simplicity and
because the PROT_* prefix was already used duplicated internally for a
set of KRB4 values.

The PROTOPT_* defines were moved up to just below the struct definition
within which they are used.