[System] UriKind.RelativeOrAbsolute workaround.

[System] UriKind.RelativeOrAbsolute workaround.

In .NET an URI constructor from "/foo" and UriKind.RelativeOrAbsolute is
relative whereas in mono it is assumed as an absolute file path.

This provides supports for an easy workaround to make mono behave as
.NET.

The workaround consists in defining DotNetRelativeOrAbsolute and using
it instead of UriKind.RelativeOrAbsolute.

DotNetRelativeOrAbsolute should be defined as follows:

static UriKind DotNetRelativeOrAbsolute = (Type.GetType ("Mono.Runtime")
== null)? UriKind.RelativeOrAbsolute : (UriKind) 300;