src/x2goclient-network-ssh.{c,h}: split out OpenSSH client process spawning into a separate function called x2goclient_network_ssh_start_sshcmd ().

This is in preparation of spawning other OpenSSH client processes (like
control processes for the master connection), so it's not a straight
copy, but something more sophisticated.

Also, the code is being reworked to not use an instance variable for the
process but to keep it local, since we're in the process of changing
this as well.