madvise.2: Document MADV_WIPEONFORK and MADV_KEEPONFORK

madvise.2: Document MADV_WIPEONFORK and MADV_KEEPONFORK

Add MADV_WIPEONFORK and MADV_KEEPONFORK documentation,
which has been merged for the 4.14 kernel.

While documenting what EINVAL means for MADV_WIPEONFORK,
I realized that MADV_FREE has the same thing going on,
so I documented EINVAL for both in the ERRORS section.

This patch documents the following kernel commit:

    commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
    Author: Rik van Riel <riel@redhat.com>
    Date:   Wed Sep 6 16:25:15 2017 -0700

        mm,fork: introduce MADV_WIPEONFORK

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Colm MacCárthaigh <colm@allcosts.net>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>