Make Frontend/Backend connection works when BE is set to use IPv6 link-local address

BackendServerIP6 has created by mythtv-setup will contain the scope Id. That scope id is only valid on the machine mythtv-setup was run on.
We must remove that scope id before attempting to use it, as otherwise the connection would fail.

IPv6 link-local addresses do cause a problem has all network interfaces have one. We attempt to determine the scope id by trying to connect to the backend using each active interfaces. Once the scope Id is found, we cache it for later use.

This fixes #10921
Fix connection to Master Backend where the master IP address is a link-local address

Similar problems as fixed in 5594a6f32e73b0519b556b63983786a01a0f2228, if MasterServerIP contains a scope Id, it will prevent frontend and slave backend to connect.

Also, the master backend detects that it is the master by comparing its IP address with the value stored in the MasterServerIP setting. The comparison is made using strings.
So if MasterServerIP is set to something like 192.168.000.001 it will fail (as we would be comparing with 192.168.0.1). IPv6 can also contains upper or lower case characters.

Add convenience methods GetMasterServerIP, GetMasterServerPort and GetMasterServerStatusPort ; which will ensure the value read is always correct.

Ref #10921
Merge branch 'master' of code.mythtv.org:mythtv