pgbouncer - pgbouncer: Explicitly detect V2 protocol request and close

Log Message:
-----------
Explicitly detect V2 protocol request and close it with proper msg.

Modified Files:
--------------
pgbouncer/include:
bouncer.h (r1.26 -> r1.27)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/include/bouncer.h?r1=1.26&r2=1.27)
pgbouncer/src:
client.c (r1.27 -> r1.28)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/client.c?r1=1.27&r2=1.28)
proto.c (r1.20 -> r1.21)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/proto.c?r1=1.20&r2=1.21)