fix: ignore Ctrl-Z when no shell can resume

Started with no job-control shell, a Ctrl-Z stopped the screen and its
launcher for good. The key now stops it only when its process group is
a shell's job, and the bar says why otherwise.

install.sh

@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="00ad7b4c93b5d69e17943e904121485da924ee681c2796a6bbd45aae6052e2b5"
+DEFAULT_INSTALL_SHA256="3e97159b3ab76eb1f3cf748b1b47de23b1acf091899eca1dff1be1d2d9406dd4"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {