MADV_DONTDUMP but for FreeBSD (MADV_NOCORE)

[zwoop]

On FreeBSD, we have MADV_NOCORE instead of MADV_DONTDUMP. I think we should refactor our code / usage of MADV_DONTDUMP such that there's a single API to be used, which then appropriately picks the right define (or no-op if not defined at all).

I'm not sure what there is for macOS.

[bryancall]

@zwoop what do you want to do with this issue?

[ywkaras]

Here's the kind of ugliness you may need for this: https://github.com/apache/trafficserver/pull/7611/files#diff-ca240207af78381db6fc12df3ce47f109d51f78e7950f74e736974832bab127aR43