Fix bug #12587 with slow startup on MS-Windows with Netlogon service.
src/fileio.c (check_existing): New function.
(make_temp_name, Ffile_exists_p, Ffile_writable_p): Call it
instead of calling 'stat', when what's needed is to check whether
a file exists. This avoids expensive system calls on MS-Windows.
src/w32.c (init_environment): Call 'check_existing' instead of 'stat'.
src/lread.c (openp) [WINDOWSNT]: Call 'access' instead of 'stat' to
determine whether a file exists and is not a directory.
src/lisp.h (check_existing): Add prototype.