Undo my 2007-02-07 change, switching back to the c-strcase module

Undo my 2007-02-07 change, switching back to the c-strcase module
introduced in the 2007-02-03 change.  Bruno Haible reported that
the 2007-02-07 change would be dangerous in Turkish if we add a
language whose name contains "i", since "i" is not lowercase "I"
in Turkish.
* bootstrap.conf (gnulib_modules): Add c-strcase.  Remove strcase.
* lib/.cvsignore: Add c-ctype.c, c-ctype.h, c-strcase.h,
c-strcasecomp.c, c-strncasecmp.c.  Remove strcasecmp.c, strncasecmp.c.
* m4/.cvsignore: Remove strcase.m4.
* src/getargs.c: Revert 2007-02-07 change, as follows.
Include c-strcase.h.
(language_argmatch): Use c_strcasecmp rather than strcasecmp.