This allows accessing and looping over the path segments in a URL
without necessarily allocating a new vector if you want them percent
decoded too (which path_segment_at_index() has an option for).
This deduplicates argument handling logic from Help and man and makes it
more modular for future use cases. The argument handling works as
before: two arguments specify section and page (in this order), one
argument specifies either a page (the first section that it's found in
is used) or a path to a manpage markdown file.
