Bug 5046: FreeBSD lacks open(2) O_DSYNC flag (#623)

Bug 5046: FreeBSD lacks open(2) O_DSYNC flag (#623)

ext_session_acl built with TrivialDB uses O_DSYNC to ensure
thread-safe manipulation of data within the TDB files in Squid
multi-process environment.

FreeBSD lacks this flag entirely. Use the O_SYNC flag as a
backup, which apparently provides file-level synchronization.
It is not clear whether this flag will prevent duplicate keys or
record overwrites in the case of process write race collisions.

NP: this appears to be FreeBSD specific. Other BSD either define
O_DSYNC or lack support for these POSIX flags entirely.