vol: FILE* to FD_t except logging and special ops

vol: FILE* to FD_t except logging and special ops

Replace all use of FILE* with FD_t.  Switch from afs_open to OS_OPEN.
In the process:

 afs_flush and afs_fsync -> OS_SYNC
 afs_stat which is used to obtain the file size -> OS_SIZE
 afs_close or close -> OS_CLOSE

Etc.   While cleaning up, use afs_stat_st macro when declaring
a struct stat because the structure and function are not guaranteed
to be the same name on all platforms.

These changes permit Windows to build the vol package without
the use of the C runtime library.

Change-Id: I9e0a7fdbeffa76c3c87aefce87ca1af495657a2f
Reviewed-on: http://gerrit.openafs.org/4149
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>