base: standardize the path and file name of core dumps

* Set aside /var/tmp/core for core dumps. Chmod it 1773.
* Set kernel.core_pattern sysctl param so core dumps are written to that
  directory, and set a file name pattern of 'core.<host>.<executable>.<pid>.<timestamp>'.
  - <host> and <timestamp> because files get scp'd around and this helps
    preserve their origin.
  - <executable> for quick visual identification.
  - <pid> so multiple core dumps from one executable don't clobber one another.
* Sweep dumps that haven't been accessed in one week.

Change-Id: If647290429877fff7f31e26ddab0e9ac783816ee