Minor refactorings and fixes in VersionMessage.

* Fixed toString() bug.
* Updated comments to match present Bitcoin Core behavior with respect to User-Agent.
* Removed constructor accepting a boolean. This changes public API but neither multibit
  nor bitcoin-wallet nor mycelium nor hive-android uses this version of constructor.
  The rationale is to avoid passing a boolean to a method as this makes code hard to read.
  Client classes updated to preserve original semantic. All tests pass.