oslib-posix: Use sysctl(2) call to resolve exec_dir on NetBSD

NetBSD 8.0(beta) ships with KERN_PROC_PATHNAME in sysctl(2).
Older NetBSD versions can use argv[0] parsing fallback.

This code section is partly shared with FreeBSD.
Signed-off-by: NKamil Rytarowski <n54@gmx.com>
Message-id: 20171028194833.23858-1-n54@gmx.com
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

util/oslib-posix.c

@@ -49,6 +49,10 @@
 #include <libutil.h>
 #endif
 
+#ifdef __NetBSD__
+#include <sys/sysctl.h>
+#endif
+
 #include "qemu/mmap-alloc.h"
 
 #ifdef CONFIG_DEBUG_STACK_USAGE
@@ -250,9 +254,14 @@ void qemu_init_exec_dir(const char *argv0)
             p = buf;
         }
     }
-#elif defined(__FreeBSD__)
+#elif defined(__FreeBSD__) \
+      || (defined(__NetBSD__) && defined(KERN_PROC_PATHNAME))
     {
+#if defined(__FreeBSD__)
         static int mib[4] = {CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1};
+#else
+        static int mib[4] = {CTL_KERN, KERN_PROC_ARGS, -1, KERN_PROC_PATHNAME};
+#endif
         size_t len = sizeof(buf) - 1;
 
         *buf = '\0';