Make poudriere jail -u work with freebsd-update
Previously, no resolv.conf were present in the jail and devfs was not mounted,
which caused DNS resolution failures and creation of strange dev/* files.
Now freebsd-update is invoked with the -b/-d/-f flags from the host.
A bug in freebsd-update (PR bin/164834) prevents it from updating the kernel, so workaround that.