(forms-read-file-filter): new hook function to

(forms-read-file-filter): new hook function to
preprocess file contents before being passed to forms mode.
(forms-write-file-filter): new hook function to preprocess file
contents before it is being saved to disk. Can be used to undo the
effects of `forms-read-file-filter'.
(forms-mode): Supply a default format if no `forms-format-list' was
specified.
Preprocess file contents using `forms-read-file-filter' and attach
`forms-write-file-filter' to the `local-write-file-hooks' of the file
buffer.
Present a friendly message if the visited file is empty or new.
(forms--intuit-from-file): New subroutine to get the number of fields
from the data file; constructs a default format list.
(forms-save-buffer): Forms mode wrapper for `save-buffer'.
(forms--change-commands, forms--exit): Use it.
(forms--update): Check for the presence of the field separator in any
of the fields.  Refuse update if found.
(forms-delete-record): Allow the last record of the file to be
deleted, even if not terminated by a newline.
(forms--local-write-file-function): Remove.  Didn't do any good.
Replaced by `forms-save-buffer'.