Remove support for (insecure) crypt authentication.

doc/src/sgml/client-auth.sgml

doc/src/sgml/protocol.sgml

doc/src/sgml/user-manag.sgml

src/backend/libpq/auth.c

src/backend/libpq/crypt.c

src/backend/libpq/hba.c

src/backend/postmaster/postmaster.c

src/include/libpq/hba.h

src/include/libpq/libpq-be.h

src/include/libpq/pqcomm.h

src/interfaces/libpq/fe-auth.c

src/interfaces/libpq/fe-connect.c

src/interfaces/libpq/libpq-int.h