vsock: implement DialContext

[Sokolmish]

Allow users to provide their own contexts for Dial() instead of using implicit Background().

I've taken the code from the previous abandoned PR and rebased it. Also I've added an option to test this functionality to the vscp utility.

Tested it this way. Stopped the VM using SIGSTOP and tried to connect to it with the vscp. With timeouts less than 3s context cancellation works as expected. With more than 3s the kernel aborts connect() itself, don't know if it's a rule though.

My test with more details:

# On the testVM (CID 43252)
socat VSOCK-LISTEN:5555,reuseaddr -   # Need to restart it after each connect

# On the hypervisor
kill -SIGSTOP <VM_PID>
./vscp -c 43252 -p 5555 -d 1s -s ./hello.txt   # Context deadline
kill -SIGCONT <VM_PID>
./vscp -c 43252 -p 5555 -d 1s -s ./hello.txt   # Works fine