Make handling of errcodes.h more consistent with other generated headers.

Make handling of errcodes.h more consistent with other generated headers.

This fixes make distprep, and seems more robust in other ways as well.
Some special handling is required because errcodes.txt is needed by
some stuff in src/port, but just by src/backend as is the case for the
other generated headers.

While I'm at it, fix a few other things that were overlooked in the
original patch.