* cpplib.c (struct directive): Const-ify name pointer and
function pointer prototype.
(validate_else, do_define, do_line, do_include, do_undef,
do_error, do_pragma, do_ident, do_if, do_xifdef, do_else,
do_elif, do_endif, do_sccs, do_assert, do_unassert,
do_warning): Const-ify second arg.
(directive_table): Mark const. Reorder entries by frequency
of usage, record statistics.