Remove some globals from nat/windows-nat.c

Remove some globals from nat/windows-nat.c

nat/windows-nat.c has a number of globals that it uses to communicate
with its clients (gdb and gdbserver).  However, if we ever want the
Windows ports to be multi-inferior, globals won't work.

This patch takes a step toward that by moving most nat/windows-nat.c
globals into a new struct windows_process_info.  Many functions are
converted to be methods on this object.

A couple of globals remain, as they are needed to truly be global due
to the way that the Windows debugging APIs work.

The clients still have a global for the current process.  That is,
this patch is a step toward the end goal, but doesn't implement the
goal itself.