rump: Stabilize rump libs and fs server

* Perform sanity check before calculating memory available to rump kernel
* Distinguish between 'Hard_context' and 'Hard_context_thread', so no dead
  threads (that will not be started) are created
* Install signal-handler for memory-resource requests. This way the fs server
  will not block forever when the quota is execeeded and a resource requests
  fails, leaving the rump kernels to perform necessary actions

Fixes #1127