Undo most of the 2007-02-03 change, switching to the strcase module

Undo most of the 2007-02-03 change, switching to the strcase module
now that gnulib strcase has been fixed.
* bootstrap.conf (gnulib_modules): Remove c-strcase.  Add strcase.
* lib/.cvsignore: Remove c-ctype.c, c-ctype.h, c-strcase.h,
c-strcasecomp.c, c-strncasecmp.c.  Add strcasecmp.c, strncasecmp.c
* m4/.cvsignore: Add strcase.m4.
* src/getargs.c: Revert 2007-02-03 change, as follows.
Don't include c-strcase.h.
(language_argmatch): Use strcasecmp rather than c_strcasecmp.
strcasecmp has "unspecified behavior" outside the POSIX locale,
but it works fine in practice if at least one argument is ASCII,
as is the case in Bison.