intl: Handle translation output codesets with suffixes [BZ #26383]

Commit 91927b7c (Rewrite iconv option parsing [BZ #19519]) did not
handle cases where the output codeset for translations (via the `gettext'
family of functions) might have a caller specified encoding suffix such as
TRANSLIT or IGNORE.  This led to a regression where translations did not
work when the codeset had a suffix.

This commit fixes the above issue by parsing any suffixes passed to
__dcigettext and adds two new test-cases to intl/tst-codeset.c to
verify correct behaviour.  The iconv-internal function __gconv_create_spec
and the static iconv-internal function gconv_destroy_spec are now visible
internally within glibc and used in intl/dcigettext.c.

(cherry picked from commit 7d4ec75e)