pgsql: Add a SHOW command to the replication command language.

Add a SHOW command to the replication command language.

This is useful infrastructure for an upcoming proposed patch to
allow the WAL segment size to be changed at initdb time; tools like
pg_basebackup need the ability to interrogate the server setting.
But it also doesn't seem like a bad thing to have independently of
that; it may find other uses in the future.

Robert Haas and Beena Emerson. (The original patch here was by
Beena, but I rewrote it to such a degree that most of the code
being committed here is mine.)

Discussion: http://postgr.es/m/CA+TgmobNo4qz06wHEmy9DszAre3dYx-WNhHSCbU9SAwf+9Ft6g@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d1ecd539477fe640455dc890216a7c1561e047b4

Modified Files
--------------
doc/src/sgml/protocol.sgml | 24 +++++++++++
src/backend/access/common/tupdesc.c | 79 ++++++++++++++++++++++++++++++++++
src/backend/replication/repl_gram.y | 22 +++++++++-
src/backend/replication/repl_scanner.l | 1 +
src/backend/replication/walsender.c | 12 ++++++
src/backend/utils/misc/guc.c | 16 +++----
src/include/access/tupdesc.h | 7 +++
7 files changed, 151 insertions(+), 10 deletions(-)