Extensive changes to provide proper support for Win32 API calls. This

Extensive changes to provide proper support for Win32 API calls.  This
new support integrates with the runtime system to provide correct
names for procedures and errors, and eliminates the need to use a
unix-style name in association with a Win32 API entity.

* Create new Win32-specific channel types.  Eliminate non-OS-specific
  channel type names.

* Introduce new file "ntapi.h" to encapsulate information about the
  Win32 API.  Rename macros for NT system calls, separating them into
  two groups: one to handle unix-style calls, the other to handle
  Win32-style calls.  Eliminate NT_<foo> aliases of C library
  procedures.  Create Win32-specific system-call names, API-call
  names, and errors.  Update all references to the old names.

* Implement OS_malloc, OS_realloc, and OS_free; change various call
  sites to use these.

* Change the low-level timer-interrupt handler to generate a GUI
  message when a Scheme interrupt is generated; this is necessary to
  unblock Scheme if it is blocked inside MsgWaitForMultipleObjects.