fix for acls with usernames that have " characters in them.

This patch seems to address all the problems with usernames with double
quotes in them. The only thing I'm worried about is what happens when you
run pg_dump now against old versions of postgres that don't use the new
quoting rules...

I've done lots of dumping and loading with usernames like [\,"- _ *<> '\,\
\" \' = \" -%,*&^% and I haven't managed to break it yet. However, whomever
commits this patch should check my logic carefully, and especially check
that I have allowed properly for malloc() space for the extra characters and
stuff.

Cheers,

Chris