Include the process identifier in the T stop responses when
multiprocess extension is enabled (i.e. prepend it to the thread
identifier). Use the exposed identifier to simplify the fork-and-follow
tests.
The LLDB client accounts for the possible PID since the multiprocess
extension support was added in b601c6719226fb83c43dae62a581e5ee08bfb169.
Sponsored by: The FreeBSD Foundation