rfc822: fix sigaction test, wean off ccan-specific talloc_set_allocator()

rfc822: fix sigaction test, wean off ccan-specific talloc_set_allocator()

The first ok() line was actually failing, because sigaction returns 0
on success, but because the program exit(0) out the abort handler,
that wasn't being seen.

So remove tap and just abort and assert our way through the test,
which I think is clearer.

We use macros to mug the talloc entry points, since talloc_set_allocator()
is not in mainline talloc.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>