Speed up most calls to 'stat' and 'lstat' on MS-Windows.

Speed up most calls to 'stat' and 'lstat' on MS-Windows.

 src/w32.c (stat_worker): If w32_stat_get_owner_group is zero, do not
 try to get accurate owner and group information from NT file
 security APIs.  This is to make most callers of 'stat' and
 'lstat', which don't need that information, much faster.
 src/dired.c (Ffile_attributes) [WINDOWSNT]: Set
 w32_stat_get_owner_group to a non-zero value, to request accurate
 owner and group information from 'lstat'.

 nt/inc/sys/stat.h: Declare w32_stat_get_owner_group.