Handle multiple Info buffers.

Handle multiple Info buffers.
(Info-tag-table-marker): Initialize to nil.
(Info-tag-table-buffer): New variable.
(Info-find-node): Don't switch buffers if already in Info mode.
Use Info-tag-table-buffer to support multiple Info buffers.
(Info-read-subfile): Don't switch to *info* buffer.
(Info-build-node-completions): Handle buffer local bindings for
Info-tag-table-marker.
(Info-search): Add a save-excursion.
(Info-mode): Make Info-tag-table-buffer buffer-local.
Make a different marker in Info-tag-table-marker for each buffer.
(Info-kill-buffer): New function, on kill-buffer-hook.