CVE-2016-7993/Clean up relative time stamp printing.

CVE-2016-7993/Clean up relative time stamp printing.

Have separate routines for signed and unsigned relative time stamps.
Have the routine for signed time stamps handle the case of -2^31, where
you can't negate it and have the result fit in a 32-bit signed integer.

There are currently no users of the signed relative time stamp routine,
but it's there in case anybody needs it (and is passing 32-bit
quantitites, so that there's a risk that a packet might contain -2^31 as
the time stamp).

Fixes an array overflow found with American Fuzzy Lop by Hanno Böck.