Remove support for protocol versions older than 3.

PostgreSQL added support for protocol version 3 in server version 7.4.
There's no need to support servers older than that. This simplifies the
code a lot, removing a lot of corner cases that haven't been properly
regression tested for a long time anyway.

Original patch by me, with some tweaks by Michael Paquier.
Remove support for ODBC versions < 3.51.

Older versions have been obsolete for a very long time by now. Removing the
support lets us get rid of a lot of code that no-one has in practice been
even compiling for a long time.

Michael Paquier
Remove references to removed KSQO and protocol options from the docs.

And some other stuff that was only relevant to very old server versions.

Michael Paquier, with some tweaks by me.
Remove hack for Windows 95 & 98.

Let's not pretend that we still support those.
Remove some remnants of ODBCVER < 3.51 support.
Remove odbcapi25w.c

All the functions in that file are obsolete, as we no longer support old
ODBC versions.

Michael Paquier
Merge info30.c and info.c

We no longer support ODBC 3.0, so these don't need to be separate anymore.
This removes some duplicated code at the bottom of each GetInfoAPI.

Michael Paquier
Avoid unnecessary round-trip to server at connection startup.

When connecting with libpq, we issued an explicit query,
"SELECT pg_client_encoding()" at the connection startup to get the
effective client_encoding. That's not really required; the server reports
that automatically right after the authentication handshake. We just have
to ask libpq for the current setting.
Remove some unused #defines
Remove support for receiving 'P' or 'B' responses from server.

Server versions >= 7.4 don't send these anymore.
Fix max identifier length stuff.

CC_get_max_idlen function didn't work correctly. It sent the "show
identifier_length" query to the backend, but assumed that the value comes
back in the command tag. It doesnt; the command tag is always "SHOW".

We fell back to the compiled in NAMEDATALEN_V73 default. But that's
off-by-one; the actual max identifier length is NAMEDATALEN - 1.
Minor code simplification.

Any compiler worth its salt will turn "strlen(constant)" into a constant,
so let's get rid of the fancy logic to precalculate those.
Remove option to disable geqo.

You can use the generic ConnSettings option for that.
Remove dead code
Change *ANSI* in the title of 32-bit Unicode driver setup dialog to *Unicode*.
Avoid conflicts of external symbols other than ODBC APis.
Allow building test suite in a different place other than the source code is
and take handling iodbc package into account.