Add deprecation notices to ce,cp sync serial drivers

And the related sconfig utility. Sync serial (e.g. E1/T1) interfaces
are obsolete, and nobody responded to several inquires on the mailing
lists about use of these drivers.

Relnotes: Yes
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23928

(cherry picked from commit 20dfe27b2d031c4000f5be94fd1db8872167a537)