KASSERT: Make runtime optionality optional

Add an option, KASSERT_PANIC_OPTIONAL, that allows runtime KASSERT()
behavior changes. When this option is not enabled, code that allows
KASSERTs to become optional is not enabled, and all violated assertions
cause termination.

The runtime KASSERT behavior was added in r243980.

One important distinction here is that panic has dead2
("attribute((noreturn))"), while kassert_panic does not. Static analyzers
like Coverity understand dead2. Without it, KASSERTs go misunderstood,
resulting in many false positives that result from violation of program
invariants.

Reviewed by: jhb, jtl, np, vangyzen
Relnotes: yes
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16835