Log an error and exit immediately when a local linked-node is configured

When a linked-node (in echocfg->Linked Nodes) contains one of the local system's FTN addresses, display/log an error and exit immediately, e.g.:
Configuration ERROR: Linked node #20 is your own address: 1:103/705

I'd perform this check in EchoCfg, but:
1. echocfg doesn't read the BBS's configuration files (where the local FTN addresses are configured), and
2. some sysops just directly edit their sbbsecho.ini file