[lldb] [llgs] Make `k` kill all processes, and fix multiple exits

Modify the behavior of the `k` packet to kill all inferiors rather than
just the current one.  The specification leaves the exact behavior
of this packet up to the implementation but since vKill is specifically
meant to be used to kill a single process, it seems logical to use `k`
to provide the alternate function of killing all of them.

Move starting stdio forwarding from the "running" response
to the packet handlers that trigger the process to start.  This avoids
attempting to start it multiple times when multiple processes are killed
on Linux which implicitly causes LLGS to receive "started" events
for all of them.  This is probably also more correct as the ability
to send "O" packets is implied by the continue-like command being issued
(and therefore the client waiting for responses) rather than the start
notification.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127500