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/sudoers.c

@@ -108,7 +108,6 @@ extern char *errorfile;
 char *login_style;
 #endif /* HAVE_BSD_AUTH_H */
 sudo_conv_t sudo_conv;
-sudo_printf_t sudo_printf;
 int sudo_mode;
 
 static char *prev_user;