x/build: enable ssh access on NetBSD builders

[bradfitz]

gomote now has ssh support, but it requires images support it.

It looks like the new netbsd-amd64-amd64 builders are not running an SSH server at all.

@bsiegert, could you make the NetBSD images have ssh running (at least on localhost) and have a gopher/gopher user with sudo access to root? Then I can configure the dashboard/builders.go bits to make gomote ssh work.

That would permit others to debug issues like #22871 more easily.

[bradfitz]

Notes to self:

$ gomote run user-bradfitz-netbsd-amd64-8branch-0 /bin/sh -c 'echo "sshd=yes" >> /etc/rc.conf'

$ gomote run user-bradfitz-netbsd-amd64-8branch-0 /bin/sh -c 'echo "PermitRootLogin without-password" >> /etc/ssh/sshd_config'

$ gomote run user-bradfitz-netbsd-amd64-8branch-0 /bin/sh -c 'mkdir /root/.ssh && chmod 0700 /root/.ssh'

$ gomote run user-bradfitz-netbsd-amd64-8branch-0 /bin/sh -c 'echo "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDQu0FJZD9tKr0ew4YWSfHDUXOjH8Fm6sZprmhEiIzvHv6atn0BmIpUhOOp650s1tRI9xQUOmtut+w0wnIMY7JB7jG8CMbctfuJB1id9JKkc8KiM2844bwSVK8mtC6lvLbArvIl/eUIFRI5+Xs33Stpw1McYrrGp/ag5a9YD0waUq3qyf3vbA+bpQZvBkIM5jtqioBTe5sorWIGipzfW+f4A1gEMvmMd4bsmuivhhPj26haaZ5JiuhsAG6AAlNZhch6Zdf7eMWovq0Xusi26W9Qpsd9sr9PpS6Vy+bjcSVnDiFeTCXOy2InXJCDSINy5RtaxRWIOlPIpTlrofS6Flcr [email protected]" >> /root/.ssh/authorized_keys'

$ gomote run user-bradfitz-netbsd-amd64-8branch-0 /etc/rc.d/sshd restart

$ ssh -i ~/.ssh/id_netbsdtest 
[email protected] 
Last login: Wed Nov 22 04:07:10 2017
NetBSD 8.0_BETA (GENERIC.201711211410Z)

Welcome to NetBSD!

This system is running a beta release of the NetBSD operating system, aimed
at stabilizing the next formal release.  It is close to formal release quality,
but may still contain bugs, even serious ones.  Please bear this in mind and
use the system with care.

You are encouraged to test this version as thoroughly as possible.  Should you  
encounter any problem, please report it back to the development team using the
send-pr(1) utility (requires a working MTA).  If yours is not properly set up,  
use the web interface at: http://www.NetBSD.org/support/send-pr.html  

Thank you for helping us test and improve this beta NetBSD release.
  
We recommend that you create a non-root account and use su(1) for root access.

[gopherbot]

Change https://golang.org/cl/81356 mentions this issue: dashboard, cmd/buildlet, env/netbsd: enable netbsd ssh, re-disable netbsd-386