Previously either the terminal client or the debugger service would
either lock main goroutine to a thread or provide a locked goroutine to
run _all_ DebuggedProcess functions in. This is unnecessary because only
ptrace functions need to be run from the same thread that originated the
PT_ATTACH request.

Here we use a specific thread-locked goroutine to service any ptrace
request. That goroutine is also responsible for the initial spawning /
attaching of the process, since it must be responsible for the PT_ATTACH
request.

Add a -headless flag which runs only the debugger server until
a SIGINT is received.

Refactor to introduce client/server separation, including a typed
client API and a HTTP REST server implementation.

Refactor the terminal to be an API consumer.