But actually, while looking at this, I understood more about what the
print_escape_nonascii flag is used for (i.e., multibyte vs unibyte
stuff), and I no longer think it makes sense for it to affect printing
the NUL byte anyway.

I propose adding a new flag print_escape_control_characters instead (see
patch #3 in the series).  I also implemented hiding the byte code
functions with text properties in #4.  It's not quite satisfactory
though, because it doesn't cover byte code functions values that are
arguments, only byte code being called.  I think printing needs to be
made more flexible in order to cleanly catch all byte code values.
Patch #5 replaces NUL bytes with "\0" in X selections (I guess it covers
w32 as well? Haven't checked yet).