Preserve ASLR vm_map flags across fork

In the most common case (fork+execve) this doesn't matter, but further
attempts to apply entropy would fail in (e.g.) a pre-fork server.

Reported by: Alfredo Mazzinghi
Reviewed by: kib, markj
Obtained from: CheriBSD
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D25966