add deprecation warning to amd

[emaste]

Summary

The man page has claimed that (built-in) amd is obsolete. Add a warning emitted by amd itself on startup as well.

% usr.sbin/amd/amd/obj/amd              
Nov 20 14:37:34 unknown amd[-1]/warn:  built-in amd in FreeBSD is deprecated
Amd configuration file (/etc/amd.conf): No such file or directory

[kevans]

We should probably add the more specific- "deprecated and will be removed before FreeBSD 13" both here and the man page.

[emaste]

extend warning as suggested by @kevans

[emaste]

update man page too

[brooks]

LGTM

[emaste]

I will commit the man page separately and MFC it in short order; I will also update it to mention that amd is available from ports (if e.g. someone doesn't want to change their existing config for some reason)