In -W style mode, check .Xr links along the full manpath because

In -W style mode, check .Xr links along the full manpath because
that is more useful for validating manuals of non-base software.
Nothing changes in -W all mode: by default for -T lint, we still
assume we want to check base system conventions, including usually
not wanting to link to non-base manual pages.

The use case, a partial idea how to handle it, and a preliminary
patch was originally presented by kn@, then refined by me.
Final patch tested and OK'ed by kn@.