- be explicit when discarding returned errors from .Close(), etc.
- remove named return values from funcs when naked return not used
- fix some "err" shadowing when redeclaration not needed
An exclude filter is basically a 'wildcard but foo', so even if a
childMayMatch, other children of a dir may not, therefore childMayMatch
does not matter, but we should not go down unless the dir is selected
for restore.
This improves restore performance by several orders of magniture by not
going through the whole tree recursively when we can anticipate that no
match will ever occur.
