mount: add support for OpenBSD

Add support for missing functions that make sense on OpenBSD.

Here's an extract of mount.active after this change:
local:
    ----------
    /:
        ----------
        device:
            /dev/sd0a
        device_uuid:
            ec8ffaec819a9077.a
        fstype:
            ffs
        major:
            4
        minor:
            0
        opts:
            - rw
            - local
            - ctime=Mon Sep  1 13:51:24 2014