Add AFS_NORETURN macro and use it

Add AFS_NORETURN macro and use it

Add the AFS_NORETURN macro which can be used with gcc compatible
compilers to indicate that a particular funciton prototype will
not return control to the caller. This both improves code quality,
and helps with static analysis.

Use this to flag afsmon_Exit, db_panic, osi_Panic, Abort, Exit
and the error handlers for osi_Assert and our local assert fn
all as being noreturn.

Change-Id: Ic6c4ae319af3d6f7665c9ee5072b556c01850982
Reviewed-on: http://gerrit.openafs.org/2134
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>