CPU times should be reported in ms

This changes CPU times to be reported in milliseconds, not ticks:
* for Linux this is done by 38032ec: this commit has been tested reasonably well;
* for Solaris this is done by e79d4e7: I haven't tested this as I don't currently have a Solaris VM to try it on although I'm reasonably sure it is OK.

In both cases the division is done slightly carefully to round rather than truncate (ie `9/10` should be `1` not `0` which is achieved by `(9 + 10/2)/10`): this probably does not matter much now but it did in an earlier version of things where I had things returning seconds.

I have also checked (by reading, not building) the Darwin and *BSD equivalents of this code and I think they all return things in ms as well.

I *haven't* checked the AIX code: I don't know AIX and don't even potentially have a machine to test it on, so I'm not sure what it does.