Windows support in pg_import_system_collations

Windows support in pg_import_system_collations

Windows can enumerate the locales that are either installed or
supported by calling EnumSystemLocalesEx(), similar to what is already
done in the READ_LOCALE_A_OUTPUT switch.  We can refactor some of the
logic already used in that switch into a new function
create_collation_from_locale().

The enumerated locales have BCP 47 shape, that is with a hyphen
between language and territory, instead of POSIX's underscore.  The
created collations will retain the BCP 47 shape, but we will also
create a POSIX alias, so xx-YY will have an xx_YY alias.

A new test collate.windows.win1252 is added that is like
collate.linux.utf8.

Author: Juan Jose Santamaria Flecha <juanjo.santamaria@gmail.com>
Reviewed-by: Dmitry Koval <d.koval@postgrespro.ru>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/0050ec23-34d9-2765-9015-98c04f0e18ac@postgrespro.ru