Add 4-byte MD5 salt.

src/backend/commands/user.c

src/backend/libpq/auth.c

src/backend/libpq/crypt.c

src/backend/libpq/md5.c

src/backend/postmaster/postmaster.c

src/include/libpq/crypt.h

src/include/libpq/libpq-be.h

src/interfaces/libpq/fe-auth.c

src/interfaces/libpq/fe-connect.c

src/interfaces/libpq/libpq-int.h

src/interfaces/odbc/connection.c