rule, set_elem: fix printing of user data

rule, set_elem: fix printing of user data

Hitherto, alphanumeric characters have been printed as-is, but anything
else was replaced by '\0'.  However, this effectively truncates the
output.  Instead, print any printable character as-is and print anything
else as a hexadecimal escape sequence:

  userdata = { \x01\x04\x01\x00\x00\x00 }

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>