Better ebug_backend_perl fallback

[plicease]

These failures:

http://matrix.cpantesters.org/?dist=Devel-ebug-HTTP+0.35

are almost entirely because ebug_backend_perl hasn't been installed into PATH. Since it is a very simple wrapper:

my $program = "@ARGV";
exec("$^X -Ilib -d:ebug::Backend $program");

we should be able to use perl directly as a backend.

Also while here, ebug_backend_perl should be using list exec to preserve quoting.