[System] Support FTP download where PWD starts with '\'.

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=13343

There are some Industrial Automation Hardware devices which return a
'\'-prefixed path to the FTP PWD command, which would break:

	System.UriFormatException: Invalid URI: The hostname could not be parsed.
	  at System.Uri.Parse (UriKind kind, System.String uriString)
	  at System.Uri.ParseUri (UriKind kind)
	  at System.Uri..ctor (System.String uriString, Boolean dontEscape)
	  at System.Uri..ctor (System.String uriString) [0x0000
	  at System.Net.FtpWebRequest.GetRemoteFolderPath (System.Uri uri)
	  at System.Net.FtpWebRequest.CWDAndSetFileName (System.Uri uri)
	  at System.Net.FtpWebRequest.ProcessMethod ()
	  at System.Net.FtpWebRequest.ProcessRequest ()
	There was an error connecting to the FTP Server.

The problem was this statement:

	Uri initial = new Uri ("ftp://dummy-host" + initial_path);

Since the FTP server was returning "\" to the PWD command,
`initial_path` was "\", and the above was equivalent to:

	Uri initial = new Uri (@"ftp://dummy-host\");

This results in the UriFormatException.

The solution is to not assume that `initial_path` will start with '/',
and instead use UriBuilder.
[System] Fix DnsTest.GetHostByName() test.

	Test Case Failures:
	1) MonoTests.System.Net.DnsTest.GetHostByName : Should raise a SocketException (assuming that 'www.unlikely.novell.com' does not exist)
	  at MonoTests.System.Net.DnsTest.GetHostByName () [0x00089] in /Users/jon/Development/mono-HEAD/mono/mcs/class/System/Test/System.Net/DnsTest.cs:212
	  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
	  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00054] in /Users/jon/Development/mono-HEAD/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:230

As it happens, 'www.unlikely.novell.com' DOES exist:

	$ ping www.unlikely.novell.com
	PING www.unlikely.novell.com (199.101.28.20): 56 data bytes
	64 bytes from 199.101.28.20: icmp_seq=0 ttl=57 time=29.399 ms

Apparently it's due to the "www." prefix; remove "www." and the site
isn't found, as expected:

	$ ping unlikely.novell.com
	ping: cannot resolve unlikely.novell.com: Unknown host

Update DnsTest.noneExistingSite so that the DnsTest.GetHostByName()
test passes.