dumpon: Fix -v causing error when configuring an encrypted dump

If -v is specified when adding a new device then a full listing of
configured devices is displayed. This requires sysctl access which
genkey()'s use of capability mode was blocking permission to access.
This leads to both confusing console spam but also incorrectly returning
an error status even if no other had been encountered.

dumpon: Sysctl get 'kern.shutdown.dumpdevname': Operation not permitted

Fix this by generating the key in a child process.

Reviewed by: markj
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D31266