remote: Make readahead_cache a C++ class

remote: Make readahead_cache a C++ class

The idea here is eliminate the get_remote_state calls from within
readahead_cache_invalidate, readahead_cache_invalidate_fd,
remote_hostio_pread_from_cache by making those functions be class
methods instead.  Later on we'll have one readahead_cache instance per
remote connection, and this change makes that easier.

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

	* remote.c (struct readahead_cache) <invalidate, invalidate_fd,
	pread>: New method declarations.
	(remote_target::open_1): Adjust.
	(readahead_cache_invalidate): Rename to ...
	(readahead_cache::invalidate): ... this, and adjust to be a class
	method.
	(readahead_cache_invalidate_fd): Rename to ...
	(readahead_cache::invalidate_fd): ... this, and adjust to be a
	class method.
	(remote_hostio_pwrite): Adjust.
	(remote_hostio_pread_from_cache): Rename to ...
	(readahead_cache::pread): ... this, and adjust to be a class
	method.
	(remote_hostio_close): Adjust.