libuutil: deobfuscate internal pointers

uu_avl and uu_list stored internal next/prev pointers and parent
pointers (unused) obfuscated (byte swapped) to hide them from a long
forgotten leak checker (No one at the 2022 OpenZFS developers meeting
could recall the history.) This would break on CHERI systems and adds
no obvious value. Rename the members, use proper types rather than
uintptr_t, and eliminate the related macros.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #14126