if you have a slow upstream link, mailing multi-megabyte
files can take minutes, and the C. DMEXCL lock can go stale,
and running the queues can cause other smtps to start up
trying to resend it.  presotto acknowledged that this is
a potential problem and that another process would be needed
to keep the lock alive, but then left the labs.

this fix recycles existing upas machinery to start a process
to keep the lock alive until the file is sent.