Use /dev/urandom, not /dev/random

/dev/random blocks if there is not enough entropy to return the
requested number of bytes, and this turns out to hang our AWS
servers. When generating random bytes for Jabber passwords, use
/dev/urandom instead. We figure that this will be "secure enough,"
since we'll have bigger problems if someone manages to get ahold
of our /dev/urandom's seed value.