Support frontend-backend protocol communication using a shm_mq.

A background worker can use pq_redirect_to_shm_mq() to direct protocol
that would normally be sent to the frontend to a shm_mq so that another
process may read them.

The receiving process may use pq_parse_errornotice() to parse an
ErrorResponse or NoticeResponse from the background worker and, if
it wishes, ThrowErrorData() to propagate the error (with or without
further modification).

Patch by me.  Review by Andres Freund.
PL/Python: Fix example

Revert "6f6b46c9c0ca3d96acbebc5499c32ee6369e1eec", which was broken.

Reported-by: Jonathan Rogers <jrogers@socialserve.com>
Add configure --enable-tap-tests option

Don't skip the TAP tests anymore when IPC::Run is not found.  This will
fail normally now.
Fix generation of INSTALL file by removing link
Fix win32setlocale.c const-related warnings.

Back-patch to 9.2, like commit db29620d4d16e08241f965ccd70d0f65883ff0de.
Make ECPG test programs depend on "ecpg$(X)", not "ecpg".

Cygwin builds require this of dependencies pertaining to pattern rules.
On Cygwin, stat("foo") in the absence of a file with that exact name can
locate foo.exe.  While GNU make uses stat() for dependencies of ordinary
rules, it uses readdir() to assess dependencies of pattern rules.
Therefore, a pattern rule dependency should match any underlying file
name exactly.  Back-patch to 9.4, where the dependency was introduced.
Re-remove dependency on the DLL of pythonxx.def file.

The reasons behind commit 0d147e43adcf5d2bff9caa073608f381a27439bf still
stand, so this reverts the non-cosmetic portion of commit
a7983e989d9cafc9cef49becfee054e34b1ed9b4.  Back-patch to 9.4, where the
latter commit first appeared.
Clarify .def file comments.
Remove dead-since-introduction pgcrypto code.

Marko Tiikkaja