The Fortuna chapter points out that saved entropy should not be backed up or
snapshotted. We don't have a lot of control over the latter as a VM guest, but
we can at least attempt to set file flags that signal "don't backup."
This is a weak signal since a lot of software might not be aware of the nodump
file flag, and some commonly used filesystems do not support it. But we might
as well set it for those that do honor it.
Testing:
```
testvm# /bin/ls -lo /var/db/entropy
total 32
-r-------- 1 operator operator nodump 4096 May 22 13:55 saved-entropy.1
-r-------- 1 operator operator - 4096 May 22 13:44 saved-entropy.2
...
```
(i.e., periodic save-entropy(8) has rolled the first file and will eventually set nodump on all files as rotation occurs).
```
testvm# /bin/ls -lo /boot/entropy
-rw------- 1 root wheel - 4096 May 22 11:08 /boot/entropy
testvm# reboot
...
Feeding entropy: .
...
testvm# /bin/ls -lo /boot/entropy
-rw------- 1 root wheel nodump 4096 May 22 13:56 /boot/entropy
```
rc.d/random updates flag.