Updates to Programs chapter of Emacs manual.

Updates to Programs chapter of Emacs manual.

* programs.texi (Program Modes): Mention modes that are not
included with Emacs.  Fix references to other manuals for tex.
Add index entry for backward-delete-char-untabify.  Mention
prog-mode-hook.
(Which Function): Use "global minor mode" terminology.
(Basic Indent, Multi-line Indent): Refer to previous descriptions
in Indentation chapter to avoid duplication.
(Expressions): Copyedit.
(Matching): Document Electric Pair mode.

* ack.texi (Acknowledgments):
* rmail.texi (Movemail, Other Mailbox Formats):
* frames.texi (Frames): Don't capitalize "Unix".