Avoid crash by stack overflow during plural expression evaluation.
* gettext-runtime/intl/plural-exp.h (enum eval_status, struct eval_result): New
types.
(plural_eval): Change return type to 'struct eval_result'.
* gettext-runtime/intl/eval-plural.h (EVAL_MAXDEPTH, OK): New macros.
(plural_eval_recurse): New function, extracted from plural_eval.
(plural_eval): Change return type to 'struct eval_result'. Invoke
plural_eval_recurse.
* gettext-runtime/intl/dcigettext.c (plural_lookup): Update.
* gettext-tools/src/plural-eval.h: Update comment.
* gettext-tools/src/msgl-check.c (plural_expression_histogram): Update.
(check_plural_eval): Update. Fail with an error message if the plural expression
evaluation produced a stack overflow.
* gettext-tools/tests/plural-3: Update.
