Share handle_load_dll and handle_unload_dll declarations

Share handle_load_dll and handle_unload_dll declarations

This changes nat/windows-nat.h to declare handle_load_dll and
handle_unload_dll.  The embedding application is required to implement
these -- while the actual code was difficult to share due to some
other differences between the two programs, sharing the declaration
lets a subsequent patch share more code that uses these as callbacks.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_nat::handle_load_dll)
	(windows_nat::handle_unload_dll): Rename.  No longer static.
	* nat/windows-nat.h (handle_load_dll, handle_unload_dll):
	Declare.

gdbserver/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* win32-low.c (windows_nat::handle_load_dll): Rename from
	handle_load_dll.  No longer static.
	(windows_nat::handle_unload_dll): Rename from handle_unload_dll.
	No longer static.