leaseshelper: store server_duid as an allocated string

leaseshelper: store server_duid as an allocated string

We either use the value from the environment variable, or learn it from
the existing lease file.

In the second case, the pointer would be pointing into the JSON object
of the first lease with a DUID, owned by leases_array, then
leases_array_new.

Always allocate the string instead, making obvious who should free the
string.