make assign to $^A update FmLINES

make assign to $^A update FmLINES

Currently assigning to $^A updates the string in
PL_bodytarget, but doesn't update FmLINES(PL_bodytarget).

This can cause later writes to get confused about how many
lines have been output, and was causing write.t to fail test 418
under miniperl. (Only under miniperl, because skipping some tests under
miniperl affected how $^A's content and line count got messed up).

Fix this by updating FmLINES(PL_bodytarget) when $^A is set.

(Also fixes a TODO test which was failing due to 'local $^A' in earlier
tests)