packages: fix ownership of /tmp and /dev

release/packages/runtime.ucl manually adds "/tmp" and "/dev" to the
"directories" key, which causes them to be included in the manifest
with the user/group owner of whoever built the package repository:

drwxr-xr-x 7 ivy wheel 512 Jun 1 17:24 /dev
drwxrwxrwt 10 ivy wheel 10 Jun 1 17:42 /tmp

Remove the manual entries and instead put an explicit package tag
in etc/mtree/BSD.root.dist. This also means /tmp gets the right
mode by default and we can remove the chmod from the post-install
script.

Reviewed by: manu, kevans, emaste
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D50636