i18n: Set LC_COLLATE locale category based on system locale.

i18n: Set LC_COLLATE locale category based on system locale.

LC_COLLATE controls the sort order of file names in GTK+ file
chooser dialog boxes, so we should set this locale category so as
to honor the user's preferences.

In the standard C library, only a few functions honor LC_COLLATE:
strcoll, wcscoll, strxfm, and wcsxfrm.  PSPP doesn't use any of
these functions, so this commit should not affect PSPP's use of
standard library functions.

The libunistring manual only mentions collation rules in the entries
for a few functions: u8_strcoll, u16_strcoll, u32_strcoll,
u8_normcoll, u16_normcoll, u32_normcoll, u8_casecoll, u16_casecoll
u32_casecoll.  PSPP doesn't use any of those functions either.

Reported by "ajk-eis" <ajk-eis@rvhome.de>.