C warning: improve pmop 0x0 not found in \%Regexp message

The \% is disturbing sprintf
C: special-case Coro jit

need to force reloading Coro::State::_jit, so that Coro::State::_jit
gets a proper ROOT assigned and the CV saved as such.
WIP In the boot_Coro__State step we now fall into READONLY no_modify croaks.
t/issue293.t: added