Fixed all known race conditions with starting a process in a new terminal.

We now spawn a thread to accept a unix socket connection from the inferior
when it spawns in the terminal, then we launch the process, then we get
the pid back through the unix socket, and then wait for it to SIGSTOP.

darwin-debug now clears the terminal screen and prints out the program and
arguments that are about to be launched.

llvm-svn: 116841