cmd/memtest: use getrusage() instead of /proc/self/stat.

cmd/memtest: use getrusage() instead of /proc/self/stat.

Only Linux has /proc/self/stat, so 'bup memtest' didn't work on anything
except Linux.  Unfortunately, getrusage() on *Linux* doesn't have a valid
RSS field (sigh), so we have to use /proc/self/stat as a fallback if it's
zero.

Now memtest works on MacOS as well, which means 'make test' passes again.
(It stopped passing because 'bup memtest' recently got added to one of the
tests.)

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>