1.43_06: Save empty bodyless cvs if they exist even if not defined

seperate defined &$sub from exist &$sub (thanks Dan Muey)
needed for signal handlers, prototypes declarations and cvrefs
(#251, #159, #235, #246, #270, #271, #275, #279)

$SIG{__WARN__} = sub{} behaves correctly now again.
Changes, STATUS: update
1.43_07:  Fix PVMG PV overwriting the RV, overload sub (#273)

A PVMG RV (ref) to a an attached overload sub was overwritten by an
empty PV. This also caused a "Attempt to free unreferenced scalar: SV"
Added t/issue273.t
#273: fix various regressions

#159, 259, ...
set $static on an ROK slot in any save_pv_or_rv, so that the RV is not
overwritten by the PV