EAP-pwd server: Use os_get_random() for unpredictable token

EAP-pwd server: Use os_get_random() for unpredictable token

Do not use os_random() that uses a low quality PRNG to generate the
anti-clogging token. The construction can be improved upon by replacing
it with a call to os_get_random(), which uses a high quality PRNG. While
the RFC 5931 explictly recommends not to do this ("SHOULD NOT be from a
source of random entropy"), it does still mandate unpredicability ("MUST
be unpredictable"). The anti-clogging token is most unpredictable when
it is taken from a high quality PRNG.

Signed-off-by: Nick Lowe <nick.lowe@lugatech.com>