MEDIUM: stats: Only report drain state in stats if server has SRV_ADMF_DRAIN set

MEDIUM: stats: Only report drain state in stats if server has SRV_ADMF_DRAIN set

There are some similarities between a weight of zero and the
administratively set drain state: both allow existing connections
to continue while not accepting any new ones.

However, when reporting a server state generally a distinction is made
between state=UP,weight=0 and state=DRAIN,weight=*. This patch makes
stats reporting consistent in this regard.

This patch does not alter the behaviour that if a server's weight
is zero then its stats row is blue when accessed via HTML. This remains
the case regardless of if the state is UP or DRAIN.

Signed-off-by: Simon Horman <horms@verge.net.au>