MFC: r251589

• The method introduced as part of r234898 (MFCed to stable/9 in r236076) for not altering the boot path when booting from ZFS turned out to also cause the boot path not being adjusted if booting from CD-ROM with firmware versions that do not employ the "cdrom" alias in that case. So shuffle the code around instead in order to achieve the original intent. Ideally, we shouldn't fiddle with the boot path when booting from UFS on a disk either; unfortunately, there doesn't seem to be an universal way of telling disks and CD-ROMs apart, though. [1]
• Use NULL instead of 0 for pointers.

PR: 179289 [1]