Support for KOI8U encoding

doc/src/sgml/charset.sgml

src/backend/utils/mb/Unicode/UCS_to_most.pl

src/backend/utils/mb/Unicode/koi8u_to_utf8.map

src/backend/utils/mb/Unicode/utf8_to_koi8u.map

src/backend/utils/mb/conversion_procs/Makefile

...mb/conversion_procs/utf8_and_cyrillic/utf8_and_cyrillic.c

src/backend/utils/mb/encnames.c

src/backend/utils/mb/wchar.c

src/include/mb/pg_wchar.h

src/port/chklocale.c