pg_restore: Don't allow non-positive number of jobs

pg_restore: Don't allow non-positive number of jobs

pg_restore will currently accept invalid values for the number of
parallel jobs to run (eg: -1), unlike pg_dump which does check that the
value provided is reasonable.

Worse, '-1' is actually a valid, independent, parameter (as an alias for
--single-transaction), leading to potentially completely unexpected
results from a command line such as:

  -> pg_restore -j -1

Where a user would get neither parallel jobs nor a single-transaction.

Add in validity checking of the parallel jobs option, as we already have
in pg_dump, before we try to open up the archive.  Also move the check
that we haven't been asked to run more parallel jobs than possible on
Windows to the same place, so we do all the option validity checking
before opening the archive.

Back-patch all the way, though for 9.2 we're adding the Windows-specific
check against MAXIMUM_WAIT_OBJECTS as that check wasn't back-patched
originally.

Discussion: https://www.postgresql.org/message-id/20170110044815.GC18360%40tamriel.snowman.net