dynamically adjust from byte-to-byte to byte-to-2bit shadowing

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:25_

PR 588482

byte-to-byte is faster but uses more memory. the idea is to start with
that and dynamically switch to byte-to-2bit using cache flushing (already
doing such a reset for -prctl_whitelist) if detect that memory is getting
low.

For back-w/-file specials, also dyn adjust to never-free
if running low on address space (xref PR 580017).

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=195_