pgsql: Add GET DIAGNOSTICS ... PG_CONTEXT in PL/PgSQL

Add GET DIAGNOSTICS ... PG_CONTEXT in PL/PgSQL

This adds the ability to get the call stack as a string from within a
PL/PgSQL function, which can be handy for logging to a table, or to
include in a useful message to an end-user.

Pavel Stehule, reviewed by Rushabh Lathia and rather heavily whacked
around by Stephen Frost.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/831283256796d1c20858862b568d73e505eb4a84

Modified Files
--------------
doc/src/sgml/plpgsql.sgml             |   57 +++++++++++++++++++++++++++
src/backend/utils/error/elog.c        |   69 +++++++++++++++++++++++++++++++++
src/include/utils/elog.h              |    2 +
src/pl/plpgsql/src/pl_exec.c          |   10 +++++
src/pl/plpgsql/src/pl_funcs.c         |    2 +
src/pl/plpgsql/src/pl_gram.y          |    6 +++
src/pl/plpgsql/src/pl_scanner.c       |    1 +
src/pl/plpgsql/src/plpgsql.h          |    1 +
src/test/regress/expected/plpgsql.out |   48 +++++++++++++++++++++++
src/test/regress/sql/plpgsql.sql      |   33 ++++++++++++++++
10 files changed, 229 insertions(+)