Reverse sense of if

Fix an inconsistency with makeman: some showconfig invocations ignore dupes,
others take the latest value as the final value. Notably, implied-on options
end up taking the latest value. *waves hands*

Update past __REQUIRED_OPTIOMS and OPT_ multi handling

This is still a little hacky; I have some ideas on how to make it
more Lua-ish and less roughly transcribed shell script.

Add some linting for the default option run to note duplicate values that we
encounter

Add a run_make() helper, switch from io.popen() to unistd.execp

We lose some convenience in popen using file handles and line-reading that
is already implemented as a result, but we cut out a number of trips through
/bin/sh as a result of doing this. I've pitched the thought of allowing
fbsd.exec() to capture stdout elsewhere and switching to that would perhaps be
a bit cleaner and a little more performant, but let's not rush that review.