Add a test support package which allows shared test functionality
for both the unit and integration tests.

Tests importing the proctl/test package will gain access to a special
test entrypoint which precompiles fixtures and makes them available
to tests.

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

Introduce a client error type which preserves HTTP error information
but which doesn't render the HTTP details by default.

This information is hardly going to be useful to a user of Delve and if
the user really needs information on new threads they can simply using
the provided commands to list and switch between threads.

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.

Eliminate a race condition in TestHalt where the RequestManualStop
call can precede the Continue call, causing the test to sporadically
fail.

* Ignore signals we do not care about
* Implement custom fork/exec for Darwin to convert signals to mach
  exceptions