Set proargmodes for satisfies_hash_partition.

Robert Haas <rhaas@postgresql.org>

Commit: be92769e4e63de949fe3ba29e0bf5c0a96f54ae3
Author: Robert Haas <rhaas@postgresql.org>
Date: 2017-11-17T16:53:00Z
Releases: 11.0
Set proargmodes for satisfies_hash_partition.

It appears that proargmodes should always be set for variadic
functions, but satifies_hash_partition had it as NULL.  In addition to
fixing the problem, add a regression test to guard against future
mistakes of this type.

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