Minor fix
Add client delete flag and improve selfdestruct

Add flag to client that instructs client to delete itself on disk at
beginning of execution. Adapt client selfdestruct handling to check
whether client already deleted itself before exiting.

Fixes #30
Add command to change client interval time

Add `chint' (change interval) command which lets the server change the
time the client sleeps between connection attempts between 1 second and
1 week.