Simplify windows-nat.c solib handling

Simplify windows-nat.c solib handling

Currently windows-nat.c uses struct so_list to record its local idea
of which shared libraries have been loaded.  However, many fields in
this are not needed, and furthermore I found this quite confusing at
first -- Windows actually uses solib-target and so the use of so_list
here is weird.

This patch simplifies this code by changing it to use a std::vector
and a new type that holds exactly what's needed for the Windows code.