define MAP_ANON for systems with MAP_ANONYMOUS

switch the sense of this check
Crank version on HEAD to 2.1.0  2.0 cointinues on OPENBSD_5_6 branch
tie master libressl branch to openbsd master

Added OPENBSD_BRANCH to set what branch update.sh should checkout.
conditionally build strnlen if needed.

it is only used by strndup

prodded by Sortie@