Bug: Background doesn't work as expected, should be deprecated and eventually removed

https://github.com/capistrano/sshkit/blob/6c81f53eec86443ea4f03fd87ae3350d4d4051ec/lib/sshkit/backends/abstract.rb#L59

The problem is that background simply `nohup` and backgrounds the process using the shell's job control `&`. More often than not, badly behaved daemons still block.

There's many hoops to jump through to ensure that your process behaves well when backgrounded, forked, nohupped, etc - and we can't whitelist programs that do it well, and blacklist those that can't.

Discussion is welcome, but I do think that `background()` should be removed from the DSL.