Windows: AFSRDFSProvider stack overrun

Windows: AFSRDFSProvider stack overrun

StringCchXXX functions take the number of characters not
the number of bytes.   Use StringCbXXXX functions whenever the
buffer size is being specified.

Check return codes from StringXXXXXX functions and return errors
instead of blindly continuing with a truncated string.

Allocate a larger buffer for substitution strings since they
need to handle the device path plus the target path.

FIXES 130392

Change-Id: I62ca980d145d6fef8cf771c26cd634ce1dd55b91
Reviewed-on: http://gerrit.openafs.org/6248
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>