pgsql: Don't allow creation of database with ICU locale with unsupporte

Don't allow creation of database with ICU locale with unsupported encoding

Check in CREATE DATABASE and initdb that the selected encoding is
supported by ICU.  Before, they would pass but users would later get
an error from the server when they tried to use the database.

Also document that initdb sets the encoding to UTF8 by default if the
ICU locale provider is chosen.

Author: Marina Polyakova <m.polyakova@postgrespro.ru>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://www.postgresql.org/message-id/6dd6db0984d86a51b7255ba79f111971@postgrespro.ru

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/c7db01e325a530ec38ec7ba57cd3ed32e123e33c

Modified Files
--------------
doc/src/sgml/ref/initdb.sgml      |  5 +++--
src/backend/commands/dbcommands.c |  9 +++++++--
src/bin/initdb/initdb.c           | 29 +++++++++++++++++++++++++++++
src/bin/initdb/t/001_initdb.pl    |  9 +++++++++
src/bin/scripts/t/020_createdb.pl |  9 +++++++++
5 files changed, 57 insertions(+), 4 deletions(-)