pgsql: Support ALTER SYSTEM RESET command.

Support ALTER SYSTEM RESET command.

This patch allows us to execute ALTER SYSTEM RESET command to
remove the configuration entry from postgresql.auto.conf.

Vik Fearing, reviewed by Amit Kapila and me.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7dfab04a8aad7aa5066d2625a649d407ed6b7a5f

Modified Files
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doc/src/sgml/ref/alter_system.sgml |   13 +++--
src/backend/parser/gram.y          |   47 +++++++++++------
src/backend/utils/misc/guc.c       |   98 ++++++++++++++++++++++--------------
src/bin/psql/tab-complete.c        |   19 +++++--
4 files changed, 115 insertions(+), 62 deletions(-)