bisect-runner.pl will now invoke with ./perl -Ilib if it sees a #!./perl line

bisect-runner.pl will now invoke with ./perl -Ilib if it sees a #!./perl line

If the first argument of the test case given to bisect-runner.pl is a readable
file with a #! line of ./perl or ./miniperl (only), bisect-runner.pl with
prepend ./perl -Ilib or ./miniperl -Ilib to the command sent to system.
This increases flexibility somewhat, and will avoid problems with
inconsistencies between directly running system(...), and running
system('valgrind', ...). The former accepts a #!./perl line, the latter sends
it to /bin/sh.