1) Simplify the way how an emacs version is picked when no emacs
is installed, but a user try to install an Emacs Lisp package.
Just pick up the version set as EMACS_TYPE than searching for
versions already installed etc. If the EMACS_TYPE version is
not supported by the Emacs Lisp Package, just fail. EMACS_TYPE
be default to GNU Emacs 21.
(In other words, users should set EMACS_TYPE as they want.
Otherwise GNU Emacs 21 is used.)
2) All Emacs Lisp Packages *must* prepend EMACS_PKGNAME_PREFIX to
a) the PKGNAME itself, and b) PKGNAME in its dependency lines.
EMACS_PKGNAME_PREFIX is expanded to "xemacs-" when XEmacs is
used. This keeps dependency graph of Emacs-Lisp-packages-
installed-for-XEmacs consistent.
3) Document EMACS_* variables as much as possible.
4) Provide more cookies for PLIST. Maybe utilized later.
Note that the 2) change doesn't affect the default, GNU Emacs 21
behaviour. So no version / revision bumps in this commit.
