New prototype for cfg_handler to remove the global variable *current_ppg.
The current ppg is now passed as argument to the cfg handler as follows:

typedef int (*cfg_handler)(int lineno, struct pp_globals *ppg,
				union pp_cfg_arg *arg);

New variable added in include/ppsi/pp-instance.h:

struct pp_globals_cfg {
	int cfg_items;
	int cur_ppi_n;
};

Now struct pp_globals has a new field:

struct pp_globals {
	[...]
	struct pp_globals_cfg cfg;
	[...]
};

The field cfg_items has been moved from pp_globals
to pp_globals_cfg. The configuration routine uses the field
cur_ppi_n inside pp_globals_cfg to store the current ppi being
configured. So neither the global variable *current_ppi is needed
anymore.

The helper inline function CUR_PPI has been defined to make
accesses to current ppi more readable.