Run SSH child processes under a ulimit

Martin Pool says on bug 506209:

"""
It would be interesting to run some bzr processes (import, mirror, maybe loggerhead) under a ulimit that signals them if they use too much memory, cpu, or other resources. This would allow a gradual ratcheting-down of those values as has been done with oops timeouts. For this to work, we'd have to make sure that when the process is signalled it does give enough data to allow useful debugging - at minimum a stacktrace; some memory info would be nice too.
"""

This would be interesting for any SSH child process, and so ought to be done in the general SSH server.