Fixed the computation of effective FilePathFilter.

If a @SlaveToMaster Callable gets sent to the master, and if inside its call() method the code directly/indirectly performs file system access (for example on files and directories that it didn't receive as parameters), that is not meant to be the subject of the filter check.

Rather, the check is meant for FilePath that the other side sends to us, when it points to a local file.

So it makes more sense to compute the effective FilePathFilter during the deserialization, as opposed to doing so when the actual operation is attempted.

Various FileCallable impls in FilePath fits this pattern, as it drags outer FilePath instance with itself. In addition, this change addresses other situations correctly, where FilePath is used as arguments and return values of other Callables, such as the following:

FilePath f = channel.call(new SlaveWillDoSomeComputationAndGiveMeTheResultAsFile());
f.copyTo(build.getRootDir().child("data.xml"))

"f.copyTo" happens in the context of Channel.current()==null, but we need to check that 'f' is legit and not something like
new FilePath(new File("/etc/shadow"))