Fix libzfs_load_module() for KERNEL_MODPREFIX != /S/L/E

For any KERNEL_MODPREFIX other than /System/Library/Extensions
libzfs_load_module() was failing.

In particular, this bug was causing the following error on 10.9+
when using the signed kexts:

	Failed to load ZFS module stack.
	Load the module manually by running
	'/sbin/kextload /Library/Extensions/zfs.kext' as root.