Having multiple instances of a single application start a single instance of another one

[Troy Melhase]

> The first time A starts, it should open a B process and start
> communicating with it. All other times an A instance starts it should
> simply talk with the B that already is open.

B should write its process id to a location known by both
applications.  When A starts, it should read that PID from the file
and attempt to communicate with the process having that PID.

When B starts, it should also check for the file.  If it's found and
if the PID in it is present in the process table, then B should exit.
Otherwise, it should start normally and write its own PID to the file.