[cmdline] Match user expectations for &&, ||, goto, and exit

The && and || operators should be left-associative, since that is how
they are treated in most other languages (including C and Unix
shell).  For example, in the command:

  dhcp net0 && goto dhcp_ok || echo No DHCP on net0

if the "dhcp net0" fails then the "echo" should be executed.

After an "exit" or a successful "goto", further commands on the same
line should never be executed.  For example:

  goto somewhere && echo This should never be printed
  exit 0 && echo This should never be printed
  exit 1 && echo This should never be printed

An "exit" should cause the current shell or script to terminate and
return the specified exit status to its caller.  For example:

  chain test.ipxe && echo Success || echo Failure
    [in test.ipxe]
    #!ipxe
    exit 0

should echo "Success".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/script.c

@@ -34,6 +34,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <ipxe/command.h>
 #include <ipxe/parseopt.h>
 #include <ipxe/image.h>
+#include <ipxe/shell.h>
 
 struct image_type script_image_type __image_type ( PROBE_NORMAL );
 
@@ -106,13 +107,8 @@ static int process_script ( int ( * process_line ) ( const char *line ),
  */
 static int terminate_on_exit_or_failure ( int rc ) {
 
-	/* Check and consume exit flag */
-	if ( shell_exit ) {
-		shell_exit = 0;
-		return 1;
-	}
-
-	return ( rc != 0 );
+	return ( shell_stopped ( SHELL_STOP_COMMAND_SEQUENCE ) ||
+		 ( rc != 0 ) );
 }
 
 /**
@@ -292,6 +288,9 @@ static int goto_exec ( int argc, char **argv ) {
 		return rc;
 	}
 
+	/* Terminate processing of current command */
+	shell_stop ( SHELL_STOP_COMMAND );
+
 	return 0;
 }