tlsdated: add jitter (-j).

We check for steady-state updates randomly at any time between
(base-jitter,base+jitter). The jitter is generated using libc rand(), seeded
with either some bytes from /dev/urandom (if available) or the time (if not). We
don't particularly need strong randomness here.

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>