Improved performance for _ready and @ready

The features which detect whether or not a specific drive is ready, or build
the list of drives which are ready, on my system require over 21s for a
disconnected but mapped network drive. In contrast an empty DVD drive takes
only 0.24s. I wonder if the next version could build the READY list when
starting up, and use a drive monitor thread to keep the list up to date.
@READY could then just scan the list for drive status. When SHRALIAS is
used, the list could even be kept in one of its memory-mapped files, so that
any new TCC instances would have instant access.

I am emailing this post on 2010.01.06 @ 09.58 EST.