Patch: make hints files warn about db-recno failures

Patch: make hints files warn about db-recno failures

Herewith some minor Configure and hints patches against 5.003_19:

1. Amend Configure so that MachTen's antediluvian  test, which holds that
   -1 is not an integer, does not bleat during non-blocking IO tests.  (Tom
   Christiansen posted a while back about several instances of this sort of
   thing; this patch addresses the only one I can see.  Are there
   still others?)

2. Make hints files for A/UX, FreeBSD, and MachTen give notification
   of the db-recno test failures caused by these systems' old db
   libraries.  This patch due mainly to Roderick Schertler, who implemented
   a suggestion I made.  I hope it's OK to put a shared stub in hints/.  It
   has the .shx extension so that it doesn't show up in Configure's list of
   systems for which there are hints.  It may be that the hints for some
   version of IRIX need the same tweak, but I couldn't work out which one
   from Martijn Koster's recent posting, and so left well enough alone.

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