Read host keys from environment variables

[Philipp Marek]

I now tried HostKeyAgent, but that alone doesn't work --
sshd reports 'sshd: no hostkeys available -- exiting.',
as [1] doesn't read `have_agent` which is set in line 1581.


Is adding that check good enough, or are there other things
to keep in mind as well?


Ad 1: 
[[https://github.com/openssh/openssh-portable/blob/0a561f90672b6dc6187763f3b90e47c4c4247875/sshd.c#L1671]]