Run the P4 mode check asynchronously.
Fixes #33. Fixes #34.
In detail:
* Remove user option p4-use-p4config-exclusively user option: it's no longer necessary to go to lots of trouble to speed up the P4 mode check.
* New buffer-local variables p4-vc-status and p4-vc-revision contain the information about the status of the file in Perforce.
* Remove variable p4-all-buffer-files: we keep track of Perforce status on a per-buffer basis, so there's no need to try to keep a list as well. As a consequence, all the code that tried to keep it up to date can be removed: p4-file-refresh-timer, p4-refresh-refresh-list, p4-file-refresh-timer-time, p4-kill-buffer-hook, p4-refresh-refresh-list, p4-refresh-buffers-in-files.
* Rename p4-check-p4-executable as p4-executable since its return value is the important bit, not the check.
* Remove p4-menu-add: this is now done in p4-status-update.
* Remove p4-find-file-hook, p4-detect-p4 and p4-check-mode: call p4-status-update instead.
* Remove p4-is-vc: use buffer-local variable p4-vc-revision instead.
* New function p4-update-status starts an asynchronous update of the P4 status for a buffer, and eventually calls new function p4-update-mode if it succeeds.
* New function p4-refresh-buffers brings all buffers under P4 control up to date.