Support proxying FTP over HTTPS, not just HTTP.

There is probably a PR for this, but I can't find this, or remember who
submitted it. The patch got lost in the noise of another that wasn't
ready to commit.

MFC after: 3 days
Approved by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit b68815f0886031fcccb426b85484b18a282d1b8c)