various refinements to network implementation

The main idea is to make DatagramChannel and *SocketChannel behave in
a way that more closely matches the standard, e.g. allow binding
sockets to addresses without necessarily listening on those addresses
and accept null addresses where appropriate.  It also avoids multiple
redundant DNS lookups.

This commit also implements CharBuffer and BindException, and adds the
Readable interface.
Merge pull request #212 from dicej/net

various refinements to network implementation
clean up System.getProperties and related methods

The behavior of Avian's versions of these methods was egregiously
non-standard, and there were problems with the Android implementations
as well.
Merge pull request #218 from dicej/properties

clean up System.getProperties and related methods