Re: Using defines for protocol characters

1. As was discussed, these definitions should go into 
src/include/libpq/pqcomm.h, not a new file.

2. I would prefer an underscore after PgMsg, like PqMsg_DescribeRequest, 
so it's easier to visually locate the start of the actual message name.

3. IMO, the names of the protocol messages in protocol.sgml are 
canonical.  Your patch appends "Request" and "Response" in cases where 
that is not part of the actual name.  Also, some messages are documented 
to go both ways, so this separation doesn't make sense strictly 
speaking.  Please use the names as in protocol.sgml without augmenting them.