Allow defining a user command inside a user command

Allow defining a user command inside a user command

PR gdb/11750 concerns defining a command inside a user commnad, like:

    define outer
      define inner
	echo hi\n
      end
    end

This patch adds this capability to gdb.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR gdb/11750:
	* cli/cli-script.h (enum command_control_type) <define_control>:
	New constant.
	* cli/cli-script.c (multi_line_command_p): Handle define_control.
	(build_command_line, execute_control_command_1)
	(process_next_line): Likewise.
	(do_define_command): New function, extracted from define_command.
	(define_command): Use it.

testsuite/ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR gdb/11750:
	* gdb.base/define.exp: Test defining a user command inside a user
	command.
	* gdb.base/commands.exp (define_if_without_arg_test): Test "define".