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 unconditionally result in data corruption.

Depends on PR #258.

Fixes #257.

I've tested these changes with cross compilation to windows, darwin, linux, openbsd, and freebsd.