Potpourri of fixes for issues uncovered in ubuntu 13.10.

* /run/shm is used for shm segments
* /tmp isn't a tmpfs, but rather a directory under /
* handle newly seen step-thread command format

A non-ubuntu-related fix is included, in which a thread spinning on a
spinlock inside a blocked-signal critical section livelocks rr.

Resolves #921.  Resolves #920.  Resolves #919.