Add a nd_printjnp() function

Add a nd_printjnp() function

It prints a null-padded filename (or other ASCII string), part of
the packet buffer, filtering out non-printable characters.
Stop if truncated (via GET_U_1/longjmp) or after n bytes or before
the null char, whichever occurs first.
The suffix comes from: j:longJmp, n:after N bytes, p:null-Padded.