[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/include/ipxe/command.h

@@ -23,6 +23,4 @@ struct command {
 
 #define __command __table_entry ( COMMANDS, 01 )
 
-extern int shell_exit;
-
 #endif /* _IPXE_COMMAND_H */

src/include/ipxe/shell.h

@@ -9,6 +9,28 @@
 
 FILE_LICENCE ( GPL2_OR_LATER );
 
+/** Shell stop states */
+enum shell_stop_state {
+	/** Continue processing */
+	SHELL_CONTINUE = 0,
+	/**
+	 * Stop processing current command line
+	 *
+	 * This is the stop state entered by commands that change the flow
+	 * of execution, such as "goto".
+	 */
+	SHELL_STOP_COMMAND = 1,
+	/**
+	 * Stop processing commands
+	 *
+	 * This is the stop state entered by commands that terminate
+	 * the flow of execution, such as "exit".
+	 */
+	SHELL_STOP_COMMAND_SEQUENCE = 2,
+};
+
+extern void shell_stop ( int stop );
+extern int shell_stopped ( int stop );
 extern int shell ( void );
 
 #endif /* _IPXE_SHELL_H */