ssh: Be more paranoid with host/domain names coming from the

never write a name with bad characters to a known_hosts file.

replace recently-added valid_domain() check for hostnames going to
known_hosts with a more relaxed check for bad characters.

Obtained from: OpenSSH-portable commit 445363433ba2
Obtained from: OpenSSH-portable commit 3cae9f92a318
Sponsored by: The FreeBSD Foundation