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/procaffinity.c

@@ -1,5 +1,5 @@
 /*
- * procaffinity stub.
+ * TODO (procaffinity stub)
  */
 
 #include <config.h>