* append operates on len, not cap (not a bug since len is set to cap above, but let's avoid the confusion)
* no need to extend ciphertext again to cap after we made it big enough
* make consistent use of ciphertext[:ivSize] vs iv[:]
* make all input problems errors and impossible/catastrophic cases panics
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.
