Fix bug in DAP handling of 'pause' requests

While working on cancellation, I noticed that a DAP 'pause' request
would set the "do not emit the continue" flag.  This meant that a
subsequent request that should provoke a 'continue' event would
instead suppress the event.

I then tried writing a more obvious test case for this, involving an
inferior call -- and discovered that gdb.events.cont does not fire for
an inferior call.

This patch installs a new event listener for gdb.events.inferior_call
and arranges for this to emit continue and stop events when
appropriate.  It also fixes the original bug, by adding a check to
exec_and_expect_stop.