Revert "Start ssh with -f instead of -n to avoid ssh hangs."

This reverts commit f002e3bb30.

That commit appears to have created a race between ssh restoring
termios state and mosh-client initializing termios.

Probably fixes #844.

scripts/mosh.pl

@@ -346,7 +346,7 @@ die "$0: fork: $!\n" unless ( defined $pid );
 if ( $pid == 0 ) { # child
   open(STDERR, ">&STDOUT") or die;
 
-  my @sshopts = ( '-f' );
+  my @sshopts = ( '-n' );
   if ($ssh_pty) {
       push @sshopts, '-tt';
   }