Linux: Improve proxy handling

While the proxy settings (env vars http_proxy, .. in /etc/environment ) are used by git and communication with Bitbucket-Server, they are not recognized by SmartGit's "Check for New Version...". The option "Use following proxy..." could be used to "Check for New Version..." but not for accessing Bitbucket on an internal network, because there isn't a "no_proxy" option in SmartGit.