sio: introduce and use sio_snprintf()

sio_strfaddr() can't be used in the places where static buffer
is not acceptable - in any code which wants to push the value to
Lua, or the address string must be long living.

The patch introduces sio_snprintf(), which does the same, but
saves the result into a provided buffer with a limited size.

In the Lua C code the patch saves the address string on the stack
which makes it safe against Lua GC interruptions.

Part of #5632