Using a module definition file gives better support to handle symbol
decorations. This change adds ssh-lsa.def to define the functions
exported from ssh-lsa.dll.
Misc. changes to ssh-lsa project (warning level, sdl check, calling
convention).
The changes necessary to support visual studio fall into 2 basic groups.
1) reordering of header files, and 2) changes to support syntax
differences between mingw and visual studio and 3). All changes are
conditional upon the definition of the macro __VS_BUILD__ .