Fix typo.
Prompt for login when async command fails due to logged-out user.
(Work in progress towards #7. The commands p4-form-command, p4-form-commit, and p4-do-notify still need fixing.)
In detail:
* Remove p4-start-p4, p4-simple-command, p4-simple-command-and-revert-buffer.
* No need for p4-call-command-process-filter -- just let the output go into the process buffer.
* When starting a process, record the command and arguments in the buffer-local variable p4-process-args.
* In p4-call-command-process-sentinel: detect failure due to logged-out user and if so, prompt user for a password, and re-start the failed command. Run callback only if command finished successfully. Use the logic from p4-simple-command to decide whether to show the output in the minibuffer or to pop to the output buffer.
* In p4-call-command, always choose a name for the output buffer based on the command and arguments (this simplifies callers).
* New function p4-refresh-callback constructs a callback function to refresh buffers after an asynchronous command completes.
* The callback function always runs in the output buffer, so we don't need the buffer argument to p4-regexp-create-links.