Create dhclient pid directory if it doesn't exist

• Upgrading from older FreeBSD versions can result in errors
• /var/run can be a tmpfs, and this should be handled correctly

Approved by: re (gjb)
Approved by: markj
Differential: https://reviews.freebsd.org/D28843
MFC after: 2 weeks

(cherry picked from commit ce94bedf475ac17307be0d5cbc66a8abe0741bf6)