If ELinks is being linked with SSL library, use its random number
generator.
Otherwise, try /dev/urandom and /dev/prandom. If they do not work,
fall back to rand(), calling srand() only once. This fallback is
mostly interesting for the Hurd and Microsoft Windows.
BitTorrent piece selection and dom/test/html-mangle.c still use rand()
(but not srand()) directly. Those would not benefit from being
unpredictable, I think.
