Fix support for writing out edited records.

Add a cf_set_frame_edited() routine to set the record header and data
for a record; have it do all the non-GUI work, and have it set the
file's "unsaved changes" flag.

Have the GUI code just call that routine and then update the title bar
to reflect the unsaved changes.

While we're at it, unmark a no-longer-unused argument to save_record().

Change-Id: Ieb513fdf423b388519527621ecec4cf634b98caf
Reviewed-on: https://code.wireshark.org/review/4885
Reviewed-by: Guy Harris <guy@alum.mit.edu>