Bug 13379: Sign our MAR files.

Configure with --enable-signmar (build the signmar tool).
Configure with --enable-verify-mar (when updating, require a valid signature
  on the MAR file before it is applied).
Use the Tor Browser version instead of the Firefox version inside the
  MAR file info block (necessary to prevent downgrade attacks).
Use NSS on all platforms for checking MAR signatures (instead of using
  OS-native APIs, which Mozilla does on Mac OS and Windows). So that the
  NSS and NSPR libraries the updater depends on can be found at runtime, we
  add the firefox directory to the shared library search path on all platforms.