Update ember code to enforce buffer size safety

[andy31415]

Problem

#4382 highlights missing output buffer limits in ember code.
Specifically for data writes, no output buffer size is provided or validated.

Proposed Solution

We should use more BufBound or similar to ensure no buffer overflows.

Ideally we should also refactor for no method to be named with 'OR' and rely on needing to read documentation to understand (i.e. method name and args should make it obvious what it does).

[bzbarsky-apple]

In particular, this is about emAfReadOrWriteAttribute and typeSensitiveMemCopy.