Make the v3 protocol ERROR-RESPONSE inherit from BACKEND-ERROR.

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- on CL implementations that support Unix sockets, the HOST argument
   to PG-CONNECT may designate the directory containing the local
   PostgreSQL unix socket (often "/var/run/postgresql/"). The HOST
   argument is assumed to designate a local directory rather than a
   hostname when its first character is #\/. You may need to modify
   authentication options in the PostgreSQL configuration file
   pg_hba.conf to allow connections over a unix-domain socket where
   the databse username is not equal to your ident tokens. This is an
   incompatible change to previous support for unix-domain sockets
   with CMUCL (previously a HOST of NIL told pg-dot-lisp to connect
   to a unix-domain socket whose name was hardwired into the library).
   This support currently exists for SBCL, CMUCL and OpenMCL.

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layout of documentation

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Integrate Peter Van Eynde's v3 protocol support:

   - create PGCON-V2 and PGCON-V3 classes
   - PG-CONNECT attempts to connect using v3 protocol, and falls back
     to v2 protocol for older backends; return a PGCON-V2 or PGCON-V3
     object
   - PG-EXEC and FN and PG-DISCONNECT are generic functions that
     dispatch on the connection type
   - protocol code split into v2-protocol.lisp and v3-protocol.lisp

TBD: cleaning up the notification & error reporting support, and
factorizing more code between the two protocol versions.

Also split code out into multiple files:

   - large-object support
   - metainformation about databases
   - parsing and type coercion support
   - utility functions and macros

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