remote: struct remote_state, use op new, fix leaks

remote: struct remote_state, use op new, fix leaks

A bit of C++ification.  Also plugs a few leaks, since currently
nothing is freeing a few fields of remote_state.  They're now freed in
the added destructor.

gdb/ChangeLog:
2018-05-22  Pedro Alves  <palves@redhat.com>

	(struct vCont_action_support): Use bool and initialize all fields.
	(struct readahead_cache): Initialize all fields.
	(remote_state): Use bool and initialize all fields.
	(remote_state::remote_state, remote_state::~remote_state): New.
	(new_remote_state): Delete.
	(_initialize_remote): Use new to allocate remote_state.