Test line breaks in PERL5DB

Test line breaks in PERL5DB

$ PERL5DB='sub DB::DB{} die' perl5.18.1 -dle 'print "ok"'
Died.
$ PERL5DB='sub DB::DB{}
die' perl5.18.1 -dle 'print "ok"'
ok

So everything after the line break is ignored.

In bleadperl:

$ PERL5DB='sub DB::DB{} die' ./perl -dle 'print "ok"'
Died.
$ PERL5DB='sub DB::DB{}
die' ./perl -dle 'print "ok"'
Died at -e line 1.

Now the whole environment variable is used.  This was probably changed
by commit 2179133.

Let’s call it a bug fix and test it.

(The ‘at -e line 1’ is not right, but the next commit will fix it.)