vol: Windows requires binary fmode for salvager

vol: Windows requires binary fmode for salvager

In order for salvager to work properly it must read and
write data in binary mode.  On Windows, this is accomplished
by calling _set_fmode(_O_BINARY) to set the default mode for
fopen(), etc.

Change-Id: Ieb00222ceccb6f044453b60d288e2badf713e517
Reviewed-on: http://gerrit.openafs.org/3665
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>