Make generated MSVC solution file open from Windows Explorer

Make generated MSVC solution file open from Windows Explorer

In order to be able to open the generated solution file by double-
clicking it in Windows Explorer, all project files need to use DOS
line-endings and a comment about the Visual Studio version needs to
be added to the header of the solution file. This also fixes the icon
that is displayed for the solution file in Windows Explorer.
Note that opening the solution file from a running instance of Visual
Studio already worked before.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Acked-by: Marius Storm-Olsen <mstormo@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>