pgsql: Add pg_trigger_depth() function

Add pg_trigger_depth() function

This reports the depth level of triggers currently in execution, or zero
if not called from inside a trigger.

No catversion bump in this patch, but you have to initdb if you want
access to the new function.

Author: Kevin Grittner

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/74ab96a45ef6259aa6a86a781580edea8488511a

Modified Files
--------------
doc/src/sgml/func.sgml                 |    7 ++
src/backend/commands/trigger.c         |   21 ++++++-
src/include/catalog/pg_proc.h          |    3 +
src/include/commands/trigger.h         |    2 +
src/test/regress/expected/triggers.out |  117 ++++++++++++++++++++++++++++++++
src/test/regress/sql/triggers.sql      |   64 +++++++++++++++++
6 files changed, 213 insertions(+), 1 deletions(-)