zfs: rename refcount.h to zfs_refcount.h avoid conflict

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
compat/opensolaris/sys/varargs.h: Dragonfly defines __va_foo in-kernel

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>