Support for the $MINTTL environment variable in wich dnscache
caches and returns the minimum of the actual ttl and $MINTTL
The implementation has been changed a bit and a new environment variable
introduced.

$MINCACHETTL specifies the minimum time (in seconds) we will
cache a RR. However clients will be sent an ever decreasing TTL until
the RR has expired - at which time the server will need to query for
a fresh RR.

$MINTTL specifies the minimum time (in seconds) the client will see. The
server will query for a new RR when it really expires but modifies the
answer (again) for the client.

Those two environment variables can be used together but take care that
the maximum ttl for a RR can be $MINCACHETTL + $MINTTL - 1 if a client
asks for a RR one second before it's supposed to expire.
A macro guard added for the external variable mincachettl
Macro guard added for mincachettl
Merge branch 'minimum-ttl'

Conflicts:
	dnscache.c