string literal continuations in C

Per a recent thread, these patches remove string literals split with
\-escaped newlines. The first is for the message "materialize mode
required, but it is not allowed in this context" where it's more
prevalent, and we keep perpetuating it; the second is for other
messages, whose bulk is in dblink and tablefunc. I think the split is
pointless and therefore propose to push both together as a single
commit, but maybe somebody would like me to leave those contrib modules
alone.

There are many other error messages that are split with no \; I would
prefer not to have them, but maybe it would be too intrusive to change
them all. So let's do this for now and remove this one point of
ugliness.

--
Álvaro Herrera 39°49'30"S 73°17'W