pg_restore -1 vs -C and -c

Hi!

As I think has been previously noted, using pg_restore with -1 (single
transaction) is fundamentally incompatible with -C (we can't CREATE
DATABASE inside a transaction) and often incompatible with -c (we don't
do DROP IF EXISTS, so if the objects don't exist the entire restore will
fail). It does work with -c *sometimes*.

It should be possible to make it compatible with -C by moving the CREATE
DATABASE command to outside of the transaction. I have only had a quick
look at the code wrt how much work this would be. One thing that hit me
quickly: do we support multiple CREATE DATABASE statements in a single
dump file? I think not, but the format seems to allow it? If not, it
should be "fairly simple" to do.

As for -c, the solution would be to issue DROP IF EXISTS statements. Is
there any particular reason why we don't?

Finally, attached is a patch that throws a nicer error message when -C
and -1 is used together, since they're fundamentally incompatible. This
one would also be backpatchable even if we fix the above.

Comments?

//Magnus