Windows: fix built in touch

Windows:  fix built in touch

Recent versions of windows add a whole bunch of attributes above
A_ARCH.  (FILE_ATTRIBUTE_NOT_CONTENT_INDEXED was what bit be but
encryption of compression would do it).

This makes ~_A_ARCH not a good choice for testing nonwritability
of a file - so files with these new attributes just get silently ignored.

Using an explicit mask is much better.  So do that.

Change-Id: Ie176ffae1995dd50c70319e196c0d30420733ccf
Reviewed-on: http://gerrit.openafs.org/3182
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>