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

sysdeps/openbsd/nosuid.c

@@ -21,16 +21,8 @@
 
 #include <config.h>
 #include <glibtop.h>
-#include <glibtop/open.h>
 #include <glibtop/close.h>
 
-void
-glibtop_open_s (glibtop *server,
-		const char *program_name,
-		const unsigned long features,
-		const unsigned flags)
-{ }
-
 void
 glibtop_close_s (glibtop *server)
 { }