Shell: Don't start a new session in every new shell

The session should be started at a higher level, i.e the Terminal app.

Shell/GlobalState.h

@@ -36,7 +36,6 @@ struct GlobalState {
     String home;
     char ttyname[32];
     char hostname[32];
-    pid_t sid;
     uid_t uid;
     struct termios termios;
     struct termios default_termios;

Shell/main.cpp

@@ -877,7 +877,6 @@ int main(int argc, char** argv)
     }
 
     g.uid = getuid();
-    g.sid = setsid();
     tcsetpgrp(0, getpgrp());
     tcgetattr(0, &g.default_termios);
     g.termios = g.default_termios;