Not executing executable files in .d directories silently is very confusing

We had a user arrive to inform us their annoyance in IRC recently. They had created a file in post-install.d called 43-rc.local and thusly spent quite a bit of time trying to figure out why diskimage-builder wouldn't execute it despite it being +x and in the right place.

Because "." is not in the allowed_regex in dib-run-parts, we skip this file. I believe this may have been done for convenience sake, so non-script files could be stored in the .d dirs, but this is extremely confusing.

So I believe we should detect any executable files that won't be executed, and warn about them loudly. Eventually we can make that an error instead of a warning. The warning should point out the install-static element which can help with transitioning away from files stored in elements like this.