Fix missing local variable initialization

This missing initializazion made remmina hangs when connecting:
remmina does not authenticated, so libssh receives a type=3
ssh packet during ssh_channel_open_session. But type=3
packet in libssh is unhandled, so ssh_channel_open_session
hangs forever.
Update multithread (pthread) support for libssh

As explained here: http://api.libssh.org/master/libssh_tutor_threads.html
Disable "Minimize windows to tray" option. Fixes #274

Disabled the "Minimize windows to tray" option. As discussed
in #274, minimizing connection windows in the system tray
is no longer possible. It's still possible to hide the main
window by closing it. It's also still possible to start
remmina whith the main window hidden with '-i'.
Manually merged commit '71837e940cf679e7bc429bf435e9fd4b8becc590'
And removed two GCC warnings caused by 71837e940cf679e7bc429bf435e9fd4b8becc590