telnet: Add Support for MIT KRB5 build

MIT KRB5 does not support telnet. This is because telnet uses 3DES
encryption, which requires 3DES encrypted keys in the KDC database.
As MIT had deprecated and removed 3DES support about a decade ago,
telnet is not supported.

Sponsored by: The FreeBSD Foundation
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D50785