volser: let it be known that Abort() really aborts

volser: let it be known that Abort() really aborts

The compiler and static checkers can do a better job if they know that
certain functions never return.  Tell it that common.c:Abort() is such
a function.  While we're at it, let volser_internal.h provide the
declarations for this function (Log() was already there).  This makes
volser parallel to the way the same functions are declared in vol.

Change-Id: I8b684bf96866edfc9edaae126d789d245a8d2356
Reviewed-on: http://gerrit.openafs.org/5175
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>