RADIUS: Use more likely unique accounting Acct-{,Multi-}Session-Id

RADIUS: Use more likely unique accounting Acct-{,Multi-}Session-Id

Rework the Acct-Session-Id and Acct-Multi-Session-Id implementation to
give better global and temporal uniqueness. Previously, only 32-bits of
the Acct-Session-Id would contain random data, the other 32-bits would
be incremented. Previously, the Acct-Multi-Session-Id would not use
random data. Switch from two u32 variables to a single u64 for the
Acct-Session-Id and Acct-Multi-Session-Id. Do not increment, this serves
no legitimate purpose. Exclusively use os_get_random() to get quality
random numbers, do not use or mix in the time. Inherently take a
dependency on /dev/urandom working properly therefore. Remove the global
Acct-Session-Id and Acct-Multi-Session-Id values that serve no
legitimate purpose.

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