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.

include/error.h

@@ -18,6 +18,7 @@
 #define	_SUDO_ERROR_H_
 
 #include <stdarg.h>
+#include <sudo_plugin.h>
 
 /*
  * We wrap error/errorx and warn/warnx so that the same output can
@@ -170,6 +171,8 @@
     warning_restore_locale();						       \
 } while (0)
 
+extern sudo_printf_t sudo_printf;
+
 void	error2(int, const char *, ...) __printflike(2, 3) __attribute__((__noreturn__));
 void	errorx2(int, const char *, ...) __printflike(2, 3) __attribute__((__noreturn__));
 void	verror2(int, const char *, va_list ap) __attribute__((__noreturn__));