PR guile/17203

	* guile/scm-param.c (pascm_parameter_defined_p): New function.
	(gdbscm_register_parameter_x): Call it.  Raise error for pre-existing
	parameters.

	testsuite/
	* gdb.guile/scm-parameter.exp: Add tests for trying to create
	previously existing parameter, and previously ambiguously spelled
	parameter.