MINOR: pattern: make pat_ref_append() return the newly added element

MINOR: pattern: make pat_ref_append() return the newly added element

It's more convenient to return the element than to return just 0 or 1,
as the next thing we'll want to do is to act on this element! In addition
it was using variable arguments instead of consts, causing some reuse
constraints which were also addressed. This doesn't change its use as
a boolean, hence why call places were not modified.