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)
lib.optional returns a singleton or an empty list. Therefore the
argument does not need to be wrapped in a list.
An alternative patch could have used lib.optionals but seems like no
more elements are going to be added to the optional list.
I know that "devinfo" output does not currently exist, but so does "devman".
It is mentioned in the nixpkgs manual, but no derivation in nixpkgs actually uses it.
