gdbserver/windows: Ignore DLL load/unload events during child initialization.

gdbserver/windows: Ignore DLL load/unload events during child initialization.

This GDBserver patch mirrors a change made in GDB wich aims at
simplifying DLL handling during the inferior initialization
(process creation during the "run", or during an "attach").
Instead of processing each DLL load event, which is sometimes
incomplete, we ignore these events until the inferior has completed
its startup phase, and then just iterate over all DLLs via
EnumProcessModules.

As a side-effect, it fixes a small bug where win32_ensure_ntdll_loaded
was missing a 0x1000 offset in the DLL base address. This problem
should only be visible on the 64bit version of Windows 8.1, since
this is the only platform where win32_ensure_ntdll_loaded is actually
needed.

gdb/gdbserver/ChangeLog:

	* win32-low.c (win32_add_all_dlls): Renames
	win32_ensure_ntdll_loaded.  Rewrite function documentation.
	Adjust implementation to always load all DLLs.
	Add 0x1000 offset to DLL base address when calling
	win32_add_one_solib.
	(child_initialization_done): New static global.
	(do_initial_child_stuff): Set child_initialization_done to
	zero during child initialization, and 1 after.  Replace call
	to win32_ensure_ntdll_loaded by call to win32_add_all_dlls.
	Add comment.
	(match_dll_by_basename, dll_is_loaded_by_basename): Delete.
	(handle_unload_dll): Add function documentation.
	(get_child_debug_event): Ignore load and unload DLL events
	during child initialization.