* monitor.c (monitor_printable_string): New function to convert a into

* monitor.c (monitor_printable_string): New function to convert a into
a printable representation.
(monitor_error): Call error after converting string into printable
format.
(monitor_printf{,_noecho}): If EXTRA_RDEBUG is defined, convert string
into printable form before printing.
(monitor_expect): Ditto.
(monitor_read_memory{,_single}): Call monitor_error, not error.
(monitor_read_memory): Return immediately if length is 0.
* ppcbug-rom.c (init_ppc_cmds): Fill in dump_registers field, which is
now required.