Optionally mark pages from cores as de-duplicatable

Optionally mark pages from cores as de-duplicatable

 Linux supports optimistic virtual-memory-level de-duplication for
 identical pages.  By default, only enable this for compressed cores.
 Runtime options --[no-]merge-core-pages can be used to override the
 default.

 Very much experimental and a hack, but compressed cores would sometimes
 be gratuitously inefficient otherwise.  If/when support is generalized
 to more OSes, figure out how to do this right.

 Thanks to David Lichteblau for the idea.