Form commands now prompt for login when user is logged out.
Fixes #7: all commands now prompt for login if possible.
In detail:
* New function p4-process-show-output contains the logic for showing P4 command output to the user.
* Avoid use of labels (deprecated in Emacs 24.3).
* p4-form-command interface simplified. It no longer takes p4-this-buffer argument: instead, it constructs the buffer name by calling p4-process-buffer-name. It no longer takes p4-out-args argument: this was unused. Improved order of optional arguments allows more callers to use the default values.
* When there's an uncommitted form buffer, re-running the form command now always selects that buffer. (Previously this was done in an ad-hoc way for some form commands.)
* Committing a form no longer kills the buffer. (Its contents might be valuable to the user.) Instead, it is marked as committed (via the p4-form-committed buffer-local variable) and buried.