More CacheStoreProc call context to afs_fetchstore from afs_segments

More CacheStoreProc call context to afs_fetchstore from afs_segments

The loop over the dcaches is performed in afs_fetchstore now as well,
in a new routine afs_CacheStoreVCache that is called in afs_CacheStoreProc's
stead. The original afs_CacheStoreProc has largely moved to
afs_CacheStoreDCaches in afs_fetchstore.c.

Enhances readability of afs_StoreAllSegments. rxfs_storeInit() can be
performed earlier now, which is instrumental for the inclusion of
alternative protocols (which will call other storeInit() functions).

Reviewed-on: http://gerrit.openafs.org/120
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>