lib-mail: Fixed handling duplicate boundary prefixes.

lib-mail: Fixed handling duplicate boundary prefixes.

If inner MIME part had the same --boundary prefix as its parent(s) and
the MIME part body started with the inner --boundary prefix, we didn't yet
have it in the list of valid boundaries, so we thought that the outer
boundary was found and the MIME headers were truncated. But due to an extra
bug we still treated it as if it were the inner boundary, except the MIME
part sizes/offsets were set wrong.

This for example fixes a situation where FETCH [1.2.MIME] returns an extra
newline before the actual headers.