stun: Use libgcrypt to provide secure random number generation

stun: Use libgcrypt to provide secure random number generation

Previously, a custom Mersenne Twister PRNG was used, which is not
securely random. In addition, its seeding fell back to wall-clock time,
which is typically predictable.

This uses libgcrypt on Linux but retains the Windows code which uses the
Windows crypt API.

Differential Revision: https://phabricator.freedesktop.org/D1610