ssh: canonicize the host name before looking it up in the host file

Re-apply r99054 by des in 2002. This was accidentally dropped
by the update to OpenSSH 6.5p1 (r261320).

This change is actually taken from r387082 of
ports/security/openssh-portable/files/patch-ssh.c

PR: 198043
Differential Revision: https://reviews.freebsd.org/D3103
Reviewed by: des
Approved by: kib (mentor)
MFC after: 3 days
Relnotes: yes
Sponsored by: Dell Inc.