ftpd: add deprecation notice

Approved by: gordon
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26447

(cherry picked from commit 98b98ec1bc7e961c3d3468b0e481080580c902cf)
(cherry picked from commit cc9347cc79d320cac00ccf5d5f23971de2c887a1)
(cherry picked from commit 01d82151022cb1383e83c331708a787f1907b00f)
(cherry picked from commit 3ea83e94cdfa34745641dfa5f43debfdcd79e229)