CVE-2016-7983,7984/Don't use strchr() to scan packet data.

CVE-2016-7983,7984/Don't use strchr() to scan packet data.

It can't be told to stop at the end of the packet data.  Add a
fn_printztn() that prints null-terminated strings, with a length check,
and which returns the number of bytes processed, or 0 if we ran out of
data.  That means it does the scanning we need, but safely.

Use it in the TFTP and BOOTP printers.

Fixes a heap overflow found with American Fuzzy Lop by Hanno Böck.