make serial_ops const

make serial_ops const

I noticed that the serial_ops vtable is not const, but really it ought
to be.

This patch constifies it, removing the only mutable field in the
process.

Tested by rebuilding on x86-64 Fedora 18, both natively and using the
mingw cross tools.

2013-12-19  Tom Tromey  <tromey@redhat.com>

	* serial.c (serial_ops_p): New typedef.
	(serial_ops_list): Now a VEC.
	(serial_interface_lookup): Return const.  Use VEC_iterate.
	(serial_add_interface): Make parameter const.
	(serial_open): Update.
	(serial_fdopen_ops): Make 'ops' const.
	(serial_pipe): Update.
	* ser-tcp.c (_initialize_ser_tcp): Update.
	* ser-pipe.c (_initialize_ser_pipe): Update.
	* ser-unix.c (_initialize_ser_hardwire): Update.
	* ser-mingw.c (_initialize_ser_windows): Update.
	* ser-go32.c (dos_ops): Now const.  Update.
	* serial.h (struct serial) <ops>: Now const.
	(struct serial_ops) <next>: Remove.
	(serial_add_interface): Make parameter const.