Avoid variable shadowing in Executer
Use same shell_log_block everywhere

For some reason there was a duplicate version in VirtualMachine which
also logged stderr (as well as stdout). That seems like a good idea to
do everywhere, so I replaced the common one with the duplicate.
Only forward two signals to Vagrant, then give up
Add logging to Executer
Store PID of running process in Executer
Add tests for Executer's signal forwarding