make life easier for translators

This patch breaks up the long, long usage text in pg_dump and pg_restore.

Every time someone changes something in this big string a translator (for
each language) have to read through it and try to figure out what it is
that differs now from the old translation.

It's much better to split it up to one string per option (and other utils
in postgresql do that).

I also replaced gettext() with _(). I didn't find any policy saying what
is the prefered way, and both _() and gettext() are used in other files. I
changed to _() since that is what most other programs use.

--
/Dennis