volser: restructure GetNextVol and clients to remove duplicate code

volser: restructure GetNextVol and clients to remove duplicate code

There are several odd-looking but stylized loops involving GetNextVol()
which can be radically simplified if only GetNextVol() would return
a meaningful value.  Move all of the code that skips non-volume-header
files in the directory into GetNextVol and have it return a truth value
(instead of always returning zero) that indicates whether it saw
something that looks like a volume header.  Then all the odd while
loops and strcmps just collapse into while(GetNextVol(...)).

GetNextVol() had external scope, but there are no callers in the
tree that use it outside of volprocs.c, and it's not part of a
public library interface, so make it static.

While here, don't strcmp() past the end of a filename that begins with
'V' but is too short to be a valid volume name.

Change-Id: I214b33c46714959d700608b3d3718c79d3792878
Reviewed-on: http://gerrit.openafs.org/7893
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>