Change debugserver to spawn processes in their own pgrp

Change debugserver's posix_spawn() to spawn an inferior process in
its own process group, so signals from the terminal like control-c
are passed to the inferior process instead of debugserer.  In lldb's
own native-host launching, there is a LaunchInfo option
LaunchInSeparateProcessGroup, and this mirrors the most common
setting of that on Darwin systems.

Patch by Alessandro Arzilli <alessandro.arzilli@gmail.com>.

Differential Revision: https://reviews.llvm.org/D128504