docdir should be used, if specified, to resolve include files

[mojavelinux]

If the docdir attribute is set, it should be used as the relative root when resolving include files. If the base_dir is also set, its value should be used as the jail.

In general, the path resolution rules need to be clearly defined and tested for all permutations of docdir and base_dir. The rules should also account for how include directives used inside of included files in nested directories are resolved.