Bring OpenBSD closer to FreeBSD to help checking what is missing/different.
Also we make sure to use and build mountlist and fsusage from the
sysdeps/openbsd directory instead of the common one.
https://bugzilla.gnome.org/show_bug.cgi?id=723521
Move OpenBSD code into its own sysdeps directory. The changes from
bsd/ were becoming to hard to maintain as a #ifdef tree. This is
what's currently being distributed as part of OpenBSD ports.