SSH improvements

[gfrlv]

An 'umbrella' issue to collect ideas on how to improve the SSH functionality.

• Use unix sockets on remote machine (now running mycli --ssh-config-host HOST without further arguments results in an error). Partial fix in try more default socket paths #795: always use tcp on SSH connections.
• Fix fallback to port 3306 when connecting over SSH
• Don't spawn separate connection for the completion thread. Instead, reuse the existing connection and open a new Channel in it: Reuse ssh connection #869
• Allow adding SSH-related info to the prompt.
• Add SSH-related parameters to DSN ([alias_dsn] config section). We could read any command line arguments from url-encoded parameters in the alias.