Refactor the mechanism for sending interrupt packets to send them
from async thread (i.e. the same thread that sends the continue packet
preceding them and that waits for the response), rather than from
the thread requesting the interrupt. This is going to become especially
important when using the vCtrlC packet as part of the non-stop protocol,
as -- unlike the simple ^c sent in the all-stop mode -- this packet
involves an explicit reply.
Suggested by Pavel Labath in D126614.
Sponsored by: The FreeBSD Foundation