minor readability changes

i've read the code a couple of times in last
two weeks and these are some adjustments based
on that experience.
Perform share valididation at crawl time.

Code Review: http://codereview.appspot.com/192090043
Preserve unnormalized path when enumerating DFS Links Part Deux.

Attempt to fix this exception:
java.lang.IllegalArgumentException: 'other' has different root
at sun.nio.fs.WindowsPath.relativize(Unknown Source)
at sun.nio.fs.WindowsPath.relativize(Unknown Source)
at com.google.enterprise.adaptor.fs.WindowsFileDelegate.enumerateDfsLinks(WindowsFileDelegate.java:326)

Code Review: http://codereview.appspot.com/192810043