xgettext: Allow plural extraction from a single argument function

xgettext: Allow plural extraction from a single argument function

The commit 8137d2b4 was a wrong fix since both singular/plural
msgids may point to the same address for Qt4 plural forms.  This
reverts the commit and fix the original double-free problem in the
right way.  Thanks to Jesper Fehrlund for suggestions.
* gettext-tools/src/xgettext.c (arglist_parser_remember_literal):
Don't ignore plural argument even if ARGNUM1 equals to ARGNUM2.
(arglist_parser_done): Make a copy of best_cp->msgid_plural when
passing it to remember_a_message_plural, if it equals to
best_cp->msgid.  Also move code conversion logic earlier taking into
account of the ownership transfer of best_cp->msgid.
* gettext-tools/src/xgettext-12: Test a single argument function.