vol: Always use INVALID_FD to indicate an invalid fd

vol: Always use INVALID_FD to indicate an invalid fd

file descriptors on Windows are not ints and therefore
cannot be safely compared against -1.  Always use INVALID_FD
which is -1 on UNIX and INVALID_HANDLE_VALUE on Windows.

Reviewed-on: http://gerrit.openafs.org/3178
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit d21dd16789458c07e63abe021f93f656dba4e52c)

Change-Id: I99e0ef2f0c1ff23817ffb13b270bc0f04bcf12a4
Reviewed-on: http://gerrit.openafs.org/3905
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>