There's a brief race here, the process might've already exited and cleaned up
after itself. If that's the case, reading from the pidfile will harmlessly
fail. Keep those quiet.
- Add license headers to source files (years based on commit dates)
in `src/test` as well as `qa`
- Add `README.md` to `src/test/data` specifying MIT license
Fixes#3848
Reworked send.sh, so it works properly on my Mac (killall send.sh
doesn't work, because the process name is 'bash' not 'send.sh').
So now send.sh writes a .send.pid file, and invoking it as
send.sh -STOP (as the bitcoind -walletnotify) signals that PID.
