Add support for enabling/disabling individual pretty-printers.

	Add support for enabling/disabling individual pretty-printers.
	* python/py-prettyprint.c (search_pp_list): Skip disabled printers.
	* python/python-internal.h (gdbpy_enabled_cst): Declare.
	* python/python.c (gdbpy_enabled_cst): Define.
	(_initialize_python): Initialize gdbpy_enabled_cst.
	* NEWS: Add entry.

	doc/
	* gdb.texinfo (Python API): New node `Disabling Pretty-Printers'.

	testsuite/
	* gdb.python/py-prettyprint.exp: Add new test for enabled and
	disabled printers.
	* gdb.python/py-prettyprint.py (disable_lookup_function): New function.
	(enable_lookup_function): New function.