Allow CLR binaries to be passed to Process.Start

On *nix files are not executable by default where on Windows they
are. This causes problems if installing a NuGet package on *nix that
contains .exe files intended to be used by build tasks. On Windows the
build will invoke these without complaining whereas on *nix the build
will fail with "Native error: cannot find file specified".

On mono, the action that will be taken is to pass the path to the .exe
file to mono as a command-line argument. If this was done in a shell it
would succeed whether or not the .exe file has the executable bit set,
so it seems that this should also succeed within a program.

With thanks to @akoeplinger for help designing the test.

I release this under the MIT license.