Use a random interval before taking the first sample

Should improve the overall sample distribution a bit, especially during program
startup.

It does not use rand() to avoid perturbing the sequence seen by the program,
and rolls its own PRNG to avoid rand_r/rand_s portability nightmares.
Generate a separate output file for each run

Use supplied file name as the prefix, appending a suffix containing
PID, time and some random digits.  Not guaranteed to be unique, but
very unlikely to collide.
Ignore more cruft when building the distribution