Update Emacs Invocation chapter of Emacs manual.

Update Emacs Invocation chapter of Emacs manual.

* doc/emacs/cmdargs.texi (Action Arguments): No need to mention
EMACSLOADPATH.
(General Variables): Add xref to Lisp Libraries.
(Initial Options): Copyedits.
(Resume Arguments): Node deleted; emacs.bash/csh are obsolete.
(Environment): Clarify what getenv does.
(General Variables): Clarify EMACSPATH etc.  Emacs does not assume
light backgrounds on xterms.
(Misc Variables): TEMP and TMP are not Windows-specific.
(Display X): Copyedits.
(Colors X): -bd does nothing for GTK.
(Icons X): Gnome 3 doesn't use taskbars.

* doc/emacs/misc.texi (Shell): Document exec-path here.

* doc/emacs/rmail.texi (Movemail): Add xref for exec-path.