pg_restore accepts -j -1

Greetings,

For reasons which seem likely to be entirely unintentional, pg_restore
will accept a '-1' for -j:

pg_restore -j -1

This seems to result in the parallel state being NULL and so things
don't outright break, but it hardly seems likely to be what the user was
asking for- my guess is that they actually wanted "parallel, single
transaction", which we don't actually support:

-> pg_restore -j 2 -1
pg_restore: cannot specify both --single-transaction and multiple jobs

We also don't accept -1 for pg_dump:

-> pg_dump -j -1
pg_dump: invalid number of parallel jobs

If I'm missing something, please let me know, otherwise I'll plan to put
the same check into pg_restore which exists in pg_dump.

Thanks!

Stephen