Fix GC_FirstDLOpenedLinkMap() for NetBSD 6 release

(this empty commit contains only the fixed message for the previous commit)

* dyn_load.c: Include sys/param.h and dlfcn.h on NetBSD.
* dyn_load.c (GC_FirstDLOpenedLinkMap): Obtain link map using dlinfo()
on NetBSD if RTLD_DI_LINKMAP feature present (defined).