Have mosh print the ssh … mosh-server command it’s running, for better debugging discoverability

Often the first step to debugging why mosh doesn’t work is to try launching mosh-server via ssh in the same way that mosh does (as described in @eminence’s handy Debugging and Theory guide). We could make that easier and more discoverable by having mosh print the exact ssh command it’s running.

We’d probably want to tidy up our shell quoting so this output isn’t full of unnecessary quotes and quoted quotes.

$ mosh localhost
/usr/bin/mosh: Running 'ssh' '-n' '-tt' '-S' 'none' '-o' 'ProxyCommand='\''mosh'\'' '\''--family=prefer-inet'\'' --fake-proxy -- %h %p' 'localhost' '--' 'mosh-server '\''new'\'' '\''-c'\'' '\''256'\'' '\''-s'\'' '\''-l'\'' '\''LANG=en_US.UTF-8'\'' '\''-l'\'' '\''LANGUAGE=en_US'\'' '\''-l'\'' '\''LC_NUMERIC=en_US.UTF-8'\'' '\''-l'\'' '\''LC_TIME=en_US.UTF-8'\'' '\''-l'\'' '\''LC_MONETARY=en_US.UTF-8'\'' '\''-l'\'' '\''LC_PAPER=en_US.UTF-8'\'' '\''-l'\'' '\''LC_NAME=en_US.UTF-8'\'' '\''-l'\'' '\''LC_ADDRESS=en_US.UTF-8'\'' '\''-l'\'' '\''LC_TELEPHONE=en_US.UTF-8'\'' '\''-l'\'' '\''LC_MEASUREMENT=en_US.UTF-8'\'' '\''-l'\'' '\''LC_IDENTIFICATION=en_US.UTF-8'\'''
bash: mosh-server: command not found
Connection to localhost closed.
/usr/bin/mosh: Did not find mosh server startup message. (Have you installed mosh on your server?)