%expext was not functioning at all.

* src/conflicts.c (expected_conflicts): Set to -1.
(conflict_report): Use ngettext.
(conflicts_print): Check %expect and make its violation an error.
* doc/bison.texinfo (Expect Decl): Adjust.
* configure.in (AM_GNU_GETTEXT): Ask for ngettext.
* tests/regression.at (%expect not enough, %expect right)
(%expect too much): New.