Hard-coding of the kext installation path now uses KERNEL_MODPREFIX (exported through zfs_config.h). Some small cleanups.

(cherry picked from commit 578b7487c1b50408576e282a8a88ae78bddcbe12)
Signed-off-by: ilovezfs <ilovezfs@icloud.com>
Stricter checking. zfs.kext is loaded from the path specified in KERNEL_MODPREFIX but not if another zfs.kext also exists in /System/Library/Extensions

(cherry picked from commit 6eafbfba80932f3a2b4b301242a42bc59b2f2fbf)
Signed-off-by: ilovezfs <ilovezfs@icloud.com>