pgsql: Add IF NOT EXISTS for CREATE SERVER and CREATE USER MAPPING

Add IF NOT EXISTS for CREATE SERVER and CREATE USER MAPPING

There is still some inconsistency with the error messages surrounding
foreign servers. Some use the word "foreign" and some don't. My
inclination is to remove all such uses of "foreign" on the basis that
the CREATE/ALTER/DROP SERVER commands don't use the word. However, that
is left for another day. In this patch I have kept to the existing usage
in the affected commands, which omits "foreign".

Anastasia Lubennikova, reviewed by Arthur Zakirov and Ashtosh Bapat.

Discussion: http://postgr.es/m/7c2ab9b8-388a-1ce0-23a3-7acf2a0ed3c6@postgrespro.ru

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b6fb534f10e1dea17dc5641f44cc651b8d60d8f0

Modified Files
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doc/src/sgml/ref/create_server.sgml | 14 ++++++++++-
doc/src/sgml/ref/create_user_mapping.sgml | 14 ++++++++++-
src/backend/commands/foreigncmds.c | 38 ++++++++++++++++++++++++++----
src/backend/parser/gram.y | 23 ++++++++++++++++++
src/include/nodes/parsenodes.h | 2 ++
src/test/regress/expected/foreign_data.out | 6 +++++
src/test/regress/sql/foreign_data.sql | 3 +++
7 files changed, 93 insertions(+), 7 deletions(-)