UDLD: Fix an infinite loop

UDLD: Fix an infinite loop

Moreover:
Update reference from draft to RFC5171.
Add bounds and length checks.
Fix TLV length printout. It is the length of the type, length, and value
fields.
Filter out non-printable characters.
Print the Echo TLV with fn_printn(). Note: The format of this list of ID
pairs is not documented in the RFC.
Update the output of a test accordingly.
Add and use istr[] and tstr[] strings.
Comment on the TLV format.
Update some comments.