Load desktop without prompting if process is dead

Load desktop without prompting if process is dead

* doc/emacs/misc.texi (Saving Emacs Sessions): Document the new
'check' value.
* etc/NEWS: Announce the change (bug#1474).
* lisp/desktop.el (desktop-load-locked-desktop): Add new value 'check'
to load desktop file without prompting if locking Emacs process does
not exist on the local machine.  (Bug#1474)
(desktop-read): Extract function from here...
(desktop--load-locked-desktop-p): ...to here.  New function handles
the semantics of 'desktop-load-locked-desktop', including above new
value 'check'.
(desktop--emacs-pid-running-p): New function.

* test/lisp/desktop-tests.el: New file with tests for the above.