Problem:
On NetBSD, `man -w open` can return the exact manpage path, but `:Man`
may still fail when man directories cannot be discovered from `manpath
-q`, bare `man -w`, or `$MANPATH`.
Solution:
Fall back to the direct manpage lookup when directory discovery fails.
Add a test for resolving `open(2)` through `goto_tag()` without manpath
data.
97a71ab484
