Allow not erase output buffer in shell commands

Allow not erase output buffer in shell commands

* lisp/simple.el (shell-command-not-erase-buffer): New option to allow
not erasing the output buffer between shell commands.  Defaults to nil.
(shell-command-on-region): Use it.
(shell-command--save-pos-or-erase): New defun; store a buffer position
if 'shell-command-not-erase-buffer' is non-nil; otherwise
erase the output buffer of the shell command.
(shell-command, shell-command-on-region): Use it.
(shell-command--set-point-after-cmd): New defun;
if 'shell-command-not-erase-buffer' is non-nil, set point
in the output buffer to the position in 'shell-command-saved-pos'.
(shell-command-sentinel, shell-command-on-region): Use it.
* doc/emacs/misc.texi (shell-command-not-erase-buffer):
Document this feature in the manual.
; * etc/NEWS: Add entry for this new feature.
See discussion on:
http://lists.gnu.org/archive/html/emacs-devel/2016-07/msg00610.html