Fix line numbers in <<foo,<<bar

Fix line numbers in <<foo,<<bar

This used to print 9:

<<foo, <<bar
a
b
c
d
e
f
g
foo
${warn __LINE__ }
bar

even though the __LINE__ marker is on line 10.  In perl 5.18, I broke
it further, making it print 2.

This commit makes it print 10.  We just need to make sure
PL_multi_start is set correctly in scan_heredoc for a second here-doc
marker on the same line.