This is the POSIX linear congruential pseudo-random generator with
48-bits of state. It is non-secure/reproducible and thread-safe.
rand() is not thread-safe and *ahem* must not be used in VLC code.
rand_r() is thread-safe but obsoleted in POSIX.2008.

For secure random numbers, use the slower vlc_rand_bytes().
vlc_rand_bytes() can also be used to generate the seed if you are short
on idea for a 48-bits pseudo-random value.