openbsd: merge patches from ports tree

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

libgtop-sysdeps.m4

@@ -79,6 +79,8 @@ AC_DEFUN([GNOME_LIBGTOP_SYSDEPS],[
 	  libgtop_sysdeps_dir=openbsd
 	  libgtop_use_machine_h=yes
 	  libgtop_need_server=yes
+	  libgtop_sysdeps_private_mountlist=yes
+	  libgtop_sysdeps_private_fsusage=yes
 	  libgtop_postinstall='chgrp kmem $(bindir)/libgtop_server2 && chmod 2555 $(bindir)/libgtop_server2'
 	  ;;
 	freebsd*|kfreebsd*)