Preserve ASLR vm_map flags across fork.

[brooks]

Summary

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

Test Plan

A variant of this patch preserving an additional local flag
works in CheriBSD.

[kib]

sys/vm/vm_map.c

4102

() are excessive.
Also I suggest to move the line right after new_map assignment (line 4098), no need for blank lines around.

sys/vm/vm_map.h

226 ↗

(On Diff #75448)

It is single-use symbol, it is easier to read code when flags are listed explicitly in vmspace_fork().

[brooks]

• Relocate flags assignment and don't use a variable.