Add an "fn_printzp()" routine for printing null-padded strings (strings

Add an "fn_printzp()" routine for printing null-padded strings (strings
with a maximum length, where a string shorter than that length is padded
with NULs), as "fn_print()" won't handle the maximum length *and* the
snapshot length and "fn_printn()" won't stop on a null string.  Use it
where appropriate.

Always pass "snapend" to "fn_print()" and "fn_printn()" if they're
passed a pointer into the packet data; only pass NULL if they're being
handed a pointer into a buffer that's not part of the packet data.

Always check the return value of "fn_print()", "fn_printn()", and
"fn_printzp()" if they're passed "snapend", and do the appropriate
string termination and "packet truncated" indication if they return 1.