Improve shell style wrt. variable naming/boolean convention

Improve shell style wrt. variable naming/boolean convention

The mixed use of upper and lower case variables is neither obvious nor
consistent.

PRINT_ONLY is local to netstart.
ip6kernel is local to netstart.
multicast gets sourced from rc.subr(8).

1. uppercase ip6kernel as is common for global variables in base scripts
2. use the simpler true/false idiom and default with the rest of
   netstart-only variables, making it clearer that only `multicast=YES/NO'
   comes from the rc environment
3. hoist kernel feature detection such that a later diff can load the SOII
   key conditionally
4. zap obvious comment

OK aja