dwm-status-runner: Use /proc/meminfo

 - The output format of `free` has changed and I'd love to not depend on
   stuff like that.
 - MemTotal and MemAvailable are the most reliable information you can
   get. No need to know anything about kernel internals.
 - MemAvailable accounts for tmpfs and more stuff like that, so we can
   kick the hacky call to `du`.

These information is available as of Linux 3.14:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773