Copy ^/head/.arcconfig to ^/vendor/.arcconfig

Having a copy of .arcconfig in ^/vendor will allow developers to submit
reviews more easily and in a manner consistent to submitting diffs (out of the
box) via arc diff in ^/head. This change eliminates the need for setting
these values [globally] via arc set-config.

Approved by: jtl (mentor)
Differential Revision: https://reviews.freebsd.org/D18621