Allow O_CLOEXEC to be used in dbopen() flags

There is also a small portability crutch, also present in NetBSD,
to allow compiling on a system that doesn't define O_CLOEXEC.

Approved by: rpaulo (mentor)
Obtained from: NetBSD (r1.17, r1.18)
Differential Revision: https://reviews.freebsd.org/D5549