__kld_whatis is an order of magnitude faster than calling `whatis` by means of
`__fish_whatis`. (It could be even faster if we could somehow tell `string
replace` to return after the first result, since the .Nd line comes at the start
of the file.)
It still takes some ~3.5 to print descriptions for all available klds (864 under
FreeBSD 13), so we still need to decide when it's prudent to do so and when it's
not.
I'm not sure what was wrong with the old syntax, but I needed to switch
the outer quotes to ' and the inner quotes to " in order for the
completions to work when they weren't explicitly sourced.
Additionally, realized that the overload for __fish_complete_suffix can
be used to get the filtered list of kernel modules from /boot/kernel in
the initial run.
