urldata: use a curl_prot_t type for storing protocol bits

This internal-use-only storage type can be bumped to a curl_off_t once
we need to use bit 32 as the previous 'unsigned int' can no longer hold
them all then.

The websocket protocols take bit 30 and 31 so they are the last ones
that fit within 32 bits - but cannot properly be exported through APIs
since those use *signed* 32 bit types (long) in places.

Closes #9481