mk.conf(5): document rump variables

Update mk.conf(5) with the rump variables from
- lib/librumpuser/README.compileopts
- sys/rump/README.compileopts

Add cross-references back to mk.conf(5) in thos

mk.conf(5): document rump variables

Update mk.conf(5) with the rump variables from
- lib/librumpuser/README.compileopts
- sys/rump/README.compileopts

Add cross-references back to mk.conf(5) in those files.

Ensure that the Default: is listed last in a description,
for consistency with the convention in this file.

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fix typo in README.compileopts text.

Move librumpuser compile-time options into the librumpuser source
directory. Those options apply only for the in-tree implementation.