This change introduces "monitor mode", a mechanism for automatically
releasing virtual machine resources when bhyve dies, bringing us
closer towards making non-root bhyve viable.
Under this regime bhyve will create a transient virtual machine using
vmmctl's VMMCTL_CREATE_DESTROY_ON_CLOSE flag and automatically
reboot said virtual machine as long as it exits with "reboot" status.
This is done by splitting bhyve into two processes. The parent process
creates the virtual machine while the child process initializes and
runs the virtual machine. When the child exits the parent inspects
its exit status and either exits or forks again. vmmctl automatically
destroys the underlying virtual machine once the parent process dies.
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.