shared/conf-parser: allow sections to be silently ignored with new -Section syntax

shared/conf-parser: allow sections to be silently ignored with new -Section syntax

If we ignore any uknown section, we will not be able to show any
warning if a typo in a section name is made. Let's reverse our
approach, and explicitly list sections to ignore instead.

I opted to make use the same section list for this, instead of adding a second
list, because this list is passed through to many functions and adding yet
another parameter to the long signature would be very noisy.