* Process.cs: Modified StartInfo to throw ArgumentNullException when

* Process.cs: Modified StartInfo to throw ArgumentNullException when
value is set to null, instead of ArgumentException. Remove check if
file name is rooted and exists when UseShellExecute is false, and let
runtime handle this. Fixes bug #345565. When a process cannot be started, do
not access ProcessStartInfo.EnvironmentVariables since that would not allow us
to re-use the instance when changing UseShellExecute. Added argument check in
Start (ProcessStartInfo). Code formatting.
* ProcessTest.cs: Added tests for Start overloads.

svn path=/trunk/mcs/; revision=90693