Move _sudo_printf from src/conversation.c to common/sudo_printf.c.

Add sudo_printf function pointer that is initialized to _sudo_printf()
instead of requiring a sudo_conv function pointer everywhere.  The
plugin will reset sudo_printf to point to the version passed in via
the plugin open function.  Now plugin_error.c can just call sudo_printf
in all cases.  The sudoers binaries no longer need their own version
of sudo_printf.

plugins/sudoers/sudoreplay.c

@@ -179,8 +179,6 @@ struct search_node {
     } u;
 } *search_expr;
 
-sudo_conv_t sudo_conv;	/* NULL in non-plugin */
-
 #define STACK_NODE_SIZE	32
 static struct search_node *node_stack[32];
 static int stack_top;