Adds custom printf formats

The specifier %r will print the contents of errstr, installed by
default.  Any others need to be manually installed.

For example:
	register_printf_specifier('i', printf_ipaddr, printf_ipaddr_info);

See tests/printf-ext.c for some examples.

If you want to add your own, add them to parlib/printf-ext.c.  Check out
the comments for ipaddr and ipaddr_info for details.  Once you get the
hang of it, it's pretty simple.

Also, some letters can't have a specifier installed onto it, such as %I
and other special chars.