vlserver: Add flags to extent address entries

vlserver: Add flags to extent address entries

Add a "flags" field to the extent address entry so that we can store
per server bit flag information. Rename the header flags feel (and
corresponding macro) so that it's explicitly a header flag. Take
this opportunity to also fix this comment to clarify that the header
flags are not a copy of anything from the vlentry, but that they
must be at the same structure offset as the vlentry flags field (so
that something accessing an extent block as if it was a vlentry can
see what it is from the flags)

Change-Id: If8a4816418d9400fb971679c08b4777e7d83c983
Reviewed-on: http://gerrit.openafs.org/4776
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>