fix t/testplc.sh 81: wrong result to test against
BC: fix test 48: newpadlx overwrites the global curpad

defer main_start, main_root, main_cv and esp. curpad after the INIT,END blocks, so that
the main_cv curpad is always the last, and any END block pad does not overwrite it.
CC: need to check AvARRAY ix against < 0 at run-time
BC: support SVf_IsCOW (fix 6 failing bytecode tests)