pgsql: Improve error checking of CREATE COLLATION options.

Improve error checking of CREATE COLLATION options.

Check for conflicting or redundant options, as we do for most other
commands. Specifying any option more than once is at best redundant,
and quite likely indicates a bug in the user's code.

While at it, improve the error for conflicting locale options by
adding detail text (the same as for CREATE DATABASE).

Bharath Rupireddy, reviewed by Vignesh C. Some additional hacking by
me.

Discussion: https://postgr.es/m/CALj2ACWtL6fTLdyF4R_YkPtf1YEDb6FUoD5DGAki3rpD+sWqiA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ba620760c4c8ca90ff83ecf7e4d46f5ec4dabd7b

Modified Files
--------------
src/backend/commands/collationcmds.c | 17 +++++++++----
src/test/regress/expected/collate.out | 47 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/collate.sql | 21 ++++++++++++++++
3 files changed, 80 insertions(+), 5 deletions(-)