want a signal handler that logs a memory dump

[Martin Pool]

It would be nice if sending bzr eg SIGUSR1 caused it to write out to a file a record of how memory is used at the moment. (Perhaps this should be inside of an apport dump.) This should not rely on bzr having a terminal, so that we can do it for server processes, including loggerhead. Using either meliae or something simpler would work, but we don't want to allocate too much more memory just to get the dump.