a common idiom is to use -N -f with port forwards to create a dedicated ssh connection in the background and keep it separate from new sessions

-N works today.  which is to say, you get a blank screen just like you would with command line ssh.  annoying to have that window always sticking around.

-f fails because it relies on daemon/fork to release the foreground process/tty.  this doesn't work in the nacl world because we stub out both daemon & fork, and even if we did support it, it wouldn't accomplish the desired effect: you'd still have a blank window with a nacl process running.

a straw man implementation:
- we already turn the ssh command line into an argv for passing down
- if -f is the first element, remove it
- send the connection request to the background page kind of like we do for sftp today
- let the background page do the auth logic
- once connected, close the window

we won't be able to process the -f flag super reliably because it could be merged with other options (-vfN), or it could be an argument to another flag (-i -f).  requiring it at the start and as a sep flag will keep this simple (for us), but might be annoying for users.  if we ever get around to reworking the UI, this would be an option we'd split out explicitly, but until then, users will have to deal with it :/.

we might want to refactor the background page logic in case there's overlap with the sftp code.  tbd.