Remove the build number from emacs-version variable

Remove the build number from emacs-version variable

It's a largely internal detail that can confuse users.  (Bug#25590)
* lisp/version.el (emacs-build-number): New constant.
(emacs-version): Use emacs-build-number.
* lisp/loadup.el (top-level): When dumping, increment
emacs-build-number rather than emacs-version.
* src/emacs.c (emacs-version): Doc fix.
* doc/lispref/intro.texi (Version Info): Update emacs-version details.
Mention emacs-build-number.
* lisp/gnus/gnus-util.el (gnus-emacs-version):
* lisp/mail/emacsbug.el (report-emacs-bug):
* admin/admin.el (set-version): Update for emacs-version change.
; * etc/NEWS: Mention this.