NLS changes

Patchers,

Here is a patch related to NLS that

- adds .y files to gettext-files in backend/nls.mk. gram.y contains
several translatable messages that are not catched by update-po unless
the gram.c file is generated. I don't know if this is desirable but I
think it's better to have to gram.y file processed.
Maybe this part of the patch could be left out (it's the first,
trivial chunk).

- makes more translator-friendly the messages in gram.y, because a lot
of them are only a keyword away of being equal. This avoids having
the translator process the same thing several times.

- adds a couple of comments for the translator (I had to peek at the
source code to see what the %s was about, so I guess this is needed).

Also, I've noted that psql's \h messages are not translatable. Is there
a way to make them so?

I have the backend messages almost done:

$ LANG= msgfmt -c -v es.po
1757 translated messages, 29 untranslated messages.

I'll submit after I resolve a couple of translations that are not clear.

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"La victoria es para quien se atreve a estar solo"