Add option to mark TCP dispatch as unshared

Add option to mark TCP dispatch as unshared

The current dispatch code could reuse the TCP connection when
dns_dispatch_gettcp() would be used first.  This is problematic as the
dns_resolver doesn't use TCP connection sharing, but dns_request could
get the TCP stream that was created outside of the dns_request.

Add new DNS_DISPATCHOPT_UNSHARED option to dns_dispatch_createtcp() that
would prevent the TCP stream to be reused.  Use that option in the
dns_resolver call to dns_dispatch_createtcp() to prevent dns_request
from reusing the TCP connections created by dns_resolver.

Additionally, the dns_xfrin unit added TCP connection sharing for
incoming transfers.  While interleaving *xfr streams on a TCP connection
should work this should be a deliberate change and be property of the
server that can be controlled.  Additionally some level of parallel TCP
streams is desirable.  Revert to the old behaviour by removing the
dns_dispatch_gettcp() calls from dns_xfrin and use the new option to
prevent from sharing the transfer streams with dns_request.