Handle var_zuinteger and var_zuinteger_unlimited from Python

PR python/20084 points out that the Python API doesn't handle the
var_zuinteger and var_zuinteger_unlimited parameter types.

This patch adds support for these types.

Regression tested on x86-64 Fedora 26.

ChangeLog
2018-05-02  Tom Tromey  <tom@tromey.com>

	PR python/20084:
	* python/python.c (gdbpy_parameter_value): Handle var_zuinteger
	and var_zuinteger_unlimited.
	* python/py-param.c (struct parm_constant): Add PARAM_ZUINTEGER
	and PARAM_ZUINTEGER_UNLIMITED.
	(set_parameter_value): Handle var_zuinteger and
	var_zuinteger_unlimited.
	(add_setshow_generic): Likewise.
	(parmpy_init): Likewise.

doc/ChangeLog
2018-05-02  Tom Tromey  <tom@tromey.com>

	PR python/20084:
	* python.texi (Parameters In Python): Document PARAM_ZUINTEGER and
	PARAM_ZUINTEGER_UNLIMITED.

testsuite/ChangeLog
2018-05-02  Tom Tromey  <tom@tromey.com>

	PR python/20084:
	* gdb.python/py-parameter.exp: Add PARAM_ZUINTEGER and
	PARAM_ZUINTEGER_UNLIMITED tests.