Fix usages of PSLIST_ENTRY_* in the example code

Fix usage of frobbotzim_prc in the example code

Tweak wording. Use \(em for emdash.

Fix xref section.

Add passive references, intermediate between pserialize and refcount.

Discussed on tech-kern:

https://mail-index.netbsd.org/tech-kern/2016/01/24/msg020069.html

API is still experimental and likely to change. (Obvious changes:
either remove extra arguments everywhere, or shrink psref_target to a
single bit, at the expense of possibly valuable diagnostic checks.)
Should do some real testing before we use this in anger!