file:/ strings are special with some datastore options, causing them to read a
file rather than emitting the exact string. This causes a couple of problems.
1. the valid? check needs to be special on assignment, since normalization
really means normalizing the path, not playing with the value as we would do
for other types
2. there are races or simply out-of-order assignments when running commands
like 'services -p 80 -R', where the datastore option is assigned before the
file is actually written.
This is the 'easy' fix of disabling assignment validation (which we didn't have
before anyway) for types that can expect a file:/ prefix.
Spencer McIntyre