Back out V6 code, caused postmaster startup failure.

configure.in

src/backend/libpq/Makefile

src/backend/libpq/auth.c

src/backend/libpq/hba.c

src/backend/libpq/pg_hba.conf.sample

src/backend/libpq/pqcomm.c

src/backend/libpq/v6util.c

src/backend/postmaster/postmaster.c

src/include/libpq/libpq.h

src/include/libpq/pqcomm.h

src/include/libpq/v6util.h

src/interfaces/libpq/Makefile

src/interfaces/libpq/fe-connect.c