flowgrindd: add -b option to set bind address

With this patch flowgrindd will create the socket for
the XML-RPC server itself and accepts the -b option to
specify the bind address for the XML-RPC Server.

With this option it is easily possible to enable IPv6 by binding
to the IPv6 wildcard address "::" .

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
flowgrind: add IPv6 parsing for rpc address
flowgrind: don't add user-agent headers in RPCs call

Recent libxmlrpc versions add a rather long user agent.
This patch configures libxmrlpc to explicitly avoid this.
Merge remote-tracking branch 'origin/master' into rpc_ipv6
Unify the way xmlrpc-c-config is called in autoconf
flowgrind: force usage of curl for xmlrpc calls
flowgrind: don't add user-agent-headers fixup

Check for "dont_advertise" member instead of strict version check.
The version check was broken, e.g. for 1.31.1
flowgrindd: properly initialize abyss server parms
flowgrind: Give hint about possible "Bad Request" error

Older xmlrpclib versions stumble upon numeric IPv6 addresses
in the HTTP "Host:" header (because such addresses contain colons).
Unfortunately there is nothing we can do about that, nor can
we detect this upfront on the controller.

At least give a hint, when such an error occurs.