Also cleaned it up a little and made it faster.
Closes#9159 and #9271
Also changes man#extract_sect_and_name_ref to only return a single
section at a time. This fixes a bug in its usage in man#goto_tag
where get_paths would be called with multiple sections and it does
not support that.
I noticed that our tagfunc doesn't obey b:man_default_sects and
I'll fix that next.