Collapse zfsloader functionality back down into loader.

We no longer really need a separate zfsloader. It was useful when we
were first supporting ZFS and had limited ability to properly boot off
of ZFS without the special boot loader. Now that the boot loader has
matured, go the way loader.efi pioneered and just build one
binary. Change the name of the loader to load in the secondary boot
blocks to be just /boot/loader. Provide a symbolic link from zfsloader
to loader so people who have not upgraded their boot blocks are not
affected. This has the happy benefit of making coexistence easier as
well (fewer binaries in the matrix).

Discussed with: allanjude@, kevans@
RelNotes: Yes
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D16361