call-stub: internal refactor

- re-structure members of call_stub_t with new simpler layout
- easier to inspect call_stub_t contents in gdb now
- fix a bunch of double unrefs and double frees in cbk stub
- change all STACK_UNWIND to STACK_UNWIND_STRICT and thereby fixed
  a lot of bad params
- implement new API call_unwind_error() which can even be called on
  fop_XXX_stub(), and not necessarily fop_XXX_cbk_stub()

Change-Id: Idf979f14d46256af0afb9658915cc79de157b2d7
BUG: 846240
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4520
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>