kio-extras: enable man protocol

This installs the kio "man:" protocol handler, which fixes the UNIX manual
section in the KDE Help Center.

Note that kde currently parses "/etc/man.conf" manually, if `$MANPATH` is not
set, to build its man page index. (if https://bugs.kde.org/show_bug.cgi?id=404022
is addressed, the "/etc/man.conf" symlink should not be necessary anymore)