use Getopt::Long hashref for --no-fsync and --dangerous

use Getopt::Long hashref for --no-fsync and --dangerous

We can eliminate the {-no_fsync} and {-dangerous} fields by
using the Getopt::Long hashref directly.  This will make it
easier to support additional CLI options (e.g. --cow).

Our internal APIs now defaults to disabling fsync, however the
CLI tools still override that internal default to enable fsync.
Having our internals default to disabling fsync can slightly
improve test performance, since they're the main users of our
unstable internal API.