Don't declare variables as "unsigned char *" that are passed to

Don't declare variables as "unsigned char *" that are passed to
functions that take "char *" arguments.  Where such chars are
assigned to int or passed to ctype functions, explicitly cast them
to unsigned char.

For OpenBSD's clang, -Wpointer-sign has been disabled by default,
but when the parse.y code was built elsewhere, the compiler would
complain.

With help from millert@
ok benno@ deraadt@