(desktop-read): Run `desktop-not-loaded-hook' in the directory where the desktop

(desktop-read): Run `desktop-not-loaded-hook' in the directory where the desktop
file was found, as the docstring says.
(desktop-kill): Use `read-directory-name'.
 (desktop-load-locked-desktop): New option.
(desktop-read): Use it.
(desktop-truncate, desktop-outvar, desktop-restore-file-buffer): Use `when'.
(desktop-save-mode-off): New function.
(desktop-base-lock-name, desktop-not-loaded-hook): New variables.
(desktop-full-lock-name, desktop-file-modtime, desktop-owner,
desktop-claim-lock, desktop-release-lock): New functions.
(desktop-kill): Tell `desktop-save' that this is the last save.
Release the lock afterwards.
(desktop-buffer-info): New function.
(desktop-save): Use it.  Run `desktop-save-hook' where the doc says to.
Detect conflicts, and manage the lock.
(desktop-read): Detect conflicts.  Manage the lock.