Virtual Info files and nodes.

Virtual Info files and nodes.
(Info-virtual-files, Info-virtual-nodes): New variables.
(Info-current-node-virtual): New variable.
(Info-virtual-file-p, Info-virtual-fun, Info-virtual-call):
New functions.
(Info-file-supports-index-cookies): Use Info-virtual-file-p
to check for a virtual file instead of checking a fixed list
of node names.
(Info-find-file): Use Info-virtual-fun and Info-virtual-call
instead of ad-hoc processing of "dir" and (apropos history toc).
(Info-find-node-2): Use Info-virtual-fun and Info-virtual-call
instead of ad-hoc processing of "dir" and (apropos history toc).
Reread a file when moving from a virtual node.
(add-to-list)<Info-virtual-files>: Add "\\`dir\\'".
(Info-directory-toc-nodes, Info-directory-find-file)
(Info-directory-find-node): New functions.
(add-to-list)<Info-virtual-files>: Add "\\`\\*History\\*\\'".
(Info-history): Move part of code to
`Info-history-find-node'.
(Info-history-toc-nodes, Info-history-find-file)
(Info-history-find-node): New functions.
(add-to-list)<Info-virtual-nodes>: Add "\\`\\*TOC\\*\\'".
(Info-toc): Move part of code to `Info-toc-find-node'.
(Info-toc-find-node): New function.
(Info-toc-insert): Renamed from `Info-insert-toc'.  Don't insert
the current Info file name to references because now the node
"*TOC*" belongs to the same Info manual.
(Info-toc-build): Renamed from `Info-build-toc'.
(Info-toc-nodes): Rename input argument `file' to `filename'.
Use Info-virtual-fun, Info-virtual-call and Info-virtual-file-p
instead of ad-hoc processing of ("dir" apropos history toc).
(Info-index-nodes): Use Info-virtual-file-p
to check for a virtual file instead of checking a fixed list
of node names.
(Info-index-node): Add check for `Info-current-node-virtual'.
Raise `save-match-data' higher up the tree to contain
`search-forward' too (bug fix).
(add-to-list)<Info-virtual-nodes>: Add "\\`\\*Index.*\\*\\'".
(Info-virtual-index-nodes): New variable.
(Info-virtual-index-find-node, Info-virtual-index): New functions.
(add-to-list)<Info-virtual-files>: Add "\\`\\*Apropos\\*\\'".
(Info-apropos-file, Info-apropos-nodes): New variables.
(Info-apropos-toc-nodes, Info-apropos-find-file)
(Info-apropos-find-node, Info-apropos-matches): New functions.
(info-apropos): Move part of code to `Info-apropos-find-node' and
`Info-apropos-matches'.
(Info-mode-map): Bind "I" to `Info-virtual-index'.
(Info-desktop-buffer-misc-data): Use Info-virtual-file-p to check
for a virtual file instead of checking a fixed list of node names.