Fix bolt on OpenBSD.

OpenBSD does not include a UBC kernel and writes must be synchronized
with the msync(2) syscall.  In addition, the NoSync field of the DB
struct should be ignored on OpenBSD, since unlike other platforms,
missing msyncs will result in data corruption.

Depends on PR #258.

Fixes #257.
Merge pull request #259 from davecgh/jrick_msync

Fix bolt on OpenBSD