Change the linux verison of proc/native and proc/gdbserial (with
debugserver) so that they let the target process use the terminal when
delve is launched in headless mode.

Windows already worked, proc/gdbserial (with rr) already worked.
I couldn't find a way to make proc/gdbserial (with lldb-server) work.

No tests are added because I can't think of a way to test for
foregroundness of a process.

Fixes #65

* proc: Add `wd` to Launch

This change adds the `wd` arg which specify working directory of the
program.

Fixes #295

* service/debugger: Add `Wd` field to debugger.Config

This change adds the `Wd` field which specify working directory of the
program launched by debugger.

Fixes #295

* service: Add `Wd` to service.Config

This change adds the `Wd` field which specify working directory of the
program debugger will launch.

Fixes #295

* cmd/dlv: Add `Wd` flag

This change adds `Wd` flag which specify working directory of the
program which launched by debugger.

Fixes #295

* only set the Linux working directory if it is set,
stub out param in darwin and windows

* set working directory for Windows
https://godoc.org/golang.org/x/sys/windows#CreateProcess
https://msdn.microsoft.com/en-us/library/windows/desktop/ms682425(v=vs.85).aspx

* Windows workingDir must be an *uint16

* attempt to chdir on darwin via @yuntan

* proc/exec_darwin.c: fix working directory for darwin

* Add tests to check if working directory works.
* Fix darwin implementation of fork/exec, which paniced if
  child fork returned.

* cmd, service: rename Wd to WorkingDir

This version preserves the order of requests, allows the
client to switch between API versions and introduces a
way to send notifications to the client (see TODO item at:
proc/proc_linux.go:325).

Fixes #523, #571