MINOR: cfgparse: implement a simple if/elif/else/endif macro block handler

MINOR: cfgparse: implement a simple if/elif/else/endif macro block handler

Very often, especially since reg-tests, it would be desirable to be able
to conditionally comment out a config block, such as removing an SSL
binding when SSL is disabled, or enabling HTX only for certain versions,
etc.

This patch introduces a very simple nested block management which takes
".if", ".elif", ".else" and ".endif" directives to take or ignore a block.

For now the conditions are limited to empty string or "0" for false versus
a non-nul integer for true, which already suffices to test environment
variables. Still, it needs to be a bit more advanced with defines, versions
etc.

A set of ".notice", ".warning" and ".alert" statements are provided to
emit messages, often in order to provide advice about how to fix certain
conditions.