Fix the value of default-directory upon startup on MS-Windows

Fix the value of default-directory upon startup on MS-Windows

* src/w32.c (w32_get_current_directory): New function.
(GetCachedVolumeInformation, init_environment): Use it.
(w32_init_current_directory): New function.
* src/w32.h (w32_init_current_directory): Add prototype.
* src/emacs.c (main) [WINDOWSNT]: Use w32_init_current_directory
to get the accurate value of cwd.  This is needed to record
the correct directory in emacs_wd, which is now initialized
way earlier in the startup process, when init_environment was
not yet called.  For details, see the problems reported in
http://lists.gnu.org/archive/html/emacs-devel/2018-12/msg00068.html.
Reported by Angelo Graziosi <angelo.g0@libero.it>.