(Info-index-nodes): New var and fun.

(Info-index-nodes): New var and fun.
(Info-goto-index, Info-index, info-apropos)
(Info-find-emacs-command-nodes): Rewrite to use Info-index-nodes.
(Info-index): Fix docstring.  Store and restore Info-history-list.
(Info-complete-nodes): New var.
(Info-complete-menu-item): Use it.
(Info-index-node): New fun.
(Info-final-node, Info-forward-node, Info-backward-node)
(Info-build-toc, Info-try-follow-nearest-node, Info-fontify-node):
Use Info-index-node.
(Info-extract-menu-item, Info-extract-menu-counting): Set second
arg of `Info-extract-menu-node-name' to non-nil for index nodes.
(Info-find-node-2): If a node with period in its name not found,
try to find a node without the name part after period.
(Info-select-node): Call Info-fontify-node only if
Info-fontify-maximum-menu-size is not nil.
(info-apropos): Set Info-fontify-maximum-menu-size to nil.
(Info-find-emacs-command-nodes, Info-goto-emacs-command-node):
Preserve Info-history-list.
(Info-toc): Set Info-current-file.
(Info-build-toc): Move point to the beginning of the buffer.
Add main-file variable.
(Info-dir-remove-duplicates, Info-history, Info-toc, info-apropos):
Use backslashed representation of the control character ^_.