Let gdb.execute handle multi-line commands

This changes the Python API so that gdb.execute can now handle
multi-line commands, like "commands" or "define".

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

	PR python/22730:
	* NEWS: Mention gdb.execute change.
	* gdbcmd.h (execute_control_command): Don't declare.
	* python/python.c (execute_gdb_command): Use read_command_lines_1,
	execute_control_commands, execute_control_commands_to_string.
	* cli/cli-script.h (execute_control_commands)
	(execute_control_commands_to_string): Declare.
	(execute_control_command): Add from_tty parameter.
	* cli/cli-script.c (execute_control_commands)
	(execute_control_commands_to_string): New functions.
	(execute_user_command): Use execute_control_commands.
	(execute_control_command_1): Add "from_tty" parameter.  Update.
	(execute_control_command): Likewise.

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

	PR python/22730:
	* gdb.python/python.exp: Test multi-line execute.