(DEBUG, DBG, DBG2, dbg): Eliminate.
	(debug_aix_thread): New static global.
	(ptrace_check, pdc_symbol_addrs, pdc_read_regs, pdc_write_regs)
	(pdc_read_data, pdc_write_data, pdc_alloc, pdc_realloc, pdc_dealloc)
	(fetch_regs_lib, store_regs_lib, store_regs_kern): Rewrite
	invocations to DBG and DBG2 macros to test against
	``debug_aix_thread'' and call fprintf_unfiltered().
	(_initialize_aix_thread): Add new command "set debug aix-thread".