kernel - Beef up lwkt_dropmsg() API and fix deadlock in so_async_rcvd*()

* Beef up the lwkt_dropmsg() API.  The API now conditionally returns
  success (0) or an error (ENOENT).

* so_pru_rcvd_async() improperly calls lwkt_sendmsg() with a spinlock
  held.  This is not legal.  Hack up lwkt_sendmsg() a bit to resolve.
kernel - Fix panic on ptrace termination

* Fix a panic in the situation where gdb is exiting and terminating
  a ptrace, but the original parent prpocess of the process being
  debugged no longer exists.