Integrate change #15879 from maint-5.6;

Integrate change #15879 from maint-5.6;

Win32::GetLongPathName() did not return valid results if there
were "." and ".." components in the path; also fix a potential
buffer overflow if the long path happens to be longer than
MAX_PATH (this can presumably happen if they use \\?\... style
paths); add a rather limited testsuite that exercises just the
edge cases
p4raw-link: @15879 on //depot/maint-5.6/perl: a15439704ef1059bf178ec4b1820fee7b2af7173

p4raw-id: //depot/perl@15880
p4raw-branched: from //depot/maint-5.6/perl@15877 'branch in'
	t/win32/longpath.t
p4raw-integrated: from //depot/maint-5.6/perl@15877 'ignore' MANIFEST
	(@12747..) 'merge in' t/harness (@11427..) win32/win32.c
	(@13145..)