Initial support of unattended mode

[jbachorik]

This change allows running BTrace probes in unattended mode when they are not connected to the initiating client.

Having the possibility of running probes in unattended mode comes very handy especially with the recent addition of JFR event support where one can setup a probe to emit custom JFR events and disconnect and leave the application running with the probe applied.

In order to support the unattended execution this PR is adding:

• the ability to detach from the command line client
• the ability to deploy the probe in unattended mode (btrace -x <options> <pid> <probe>)
• the ability to list all active probes which can be attached to (btrace -lp <pid>)
• the ability to reconnect to a previously disconnected probes (btrace -r <probe id> <pid>) where probe id is assigned (and displayed) when the probe is first time deployed

For disconnected probes there is small message buffers which can hold up to 5000 most recent messages so when reconnecting one can get the most recent history replayed.