w3m reads the history file on startup and writes it on exit. That means
if you open multiple instances of w3m, the history file will contain the
history of the last instance closed. All other history changes are lost.
Check if the modification time of the history file has changed before
writing. If it has changed read the history file from the disk into a
new history. Push the entries that are in the current history but not in
the history file into the new history and write the new history to disk.
With this patch applied the user can configure a directory to store
temporary/cache files. The history and cookies remain in RC_DIR.
I suppose the intent of writing the history to a temp file first is to
make the actual write an atomic operation. As rename() does not work
across mount points, we need to handle the temp file for the history
different to keep this behaviour.
Add a new type for the temp history file and handle this case different
when creating a temp file.
