Second attempt at preventing zombies (#16326)

Second attempt at preventing zombies (#16326)

* Second attempt at preventing zombies

* Ensure that the pipes are closed in ssh.go
* Ensure that a cancellable context is passed up in cmd/* http requests
* Make cmd.fail return properly so defers are obeyed
* Ensure that something is sent to stdout in case of blocks here

Signed-off-by: Andrew Thornton <art27@cantab.net>
* placate lint

Signed-off-by: Andrew Thornton <art27@cantab.net>
* placate lint 2

Signed-off-by: Andrew Thornton <art27@cantab.net>
* placate lint 3

Signed-off-by: Andrew Thornton <art27@cantab.net>
* fixup

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Apply suggestions from code review

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>