Bug-fix: Support [node:<addr>@<domain>] sections in AreaFix changes.

When processing area manager netmail messages, SBBSecho can modify
(add/change) keys in the [node:*] section of the sbbsecho.ini file. Although
SBBSecho would read node sections that had a domain (e.g. 5D address) just fine
when processing area manager changes and writing those changes back to the
sbbsecho.ini, only 4D addresses (no "@domain") were used in the section naming.
This could result in duplicate node sections being created in the sbbsecho.ini
file (one with a "@domain", one without) and then "bad things" could happen
(e.g. BinkP authentication failures, packet authentication failures, etc.).

Now, when SBBSecho writes linked-node setting to the sbbsecho.ini, it first
checks for the full 5D address of the node and if that section exists, modify
it, otherwise fall-through to the 3D/4D address of the node for the sectoin
naming.