Patch for psql to avoid altering the PGresult strings

The mbvalidate() function was called on the PGresult strings, and it
changes the strings (removes unknown characters).

I've change the validation function to one that either returns a new
string when needed or NULL when the string already was validating. The
normal case is to have validating strings so in most cases no new strings
are created.

I have not included changes to avoid sending non validating strings to the
server. It's not clear what is the best way to solve it. Maybe one should
just do a stupid fix for 7.4 and validate the strings like above and just
delete non validating strings. It is possible to solve it in a better way,
but it's more complicated. We have 4 different charsets to think about in
psql. The server, the client, the terminal and the message catalog (the
last two usually matches, but not always). I would prefer to find a good
solution instead of just patching one small problem after another.

--
/Dennis