Common login control (e.g. via real name) using new find_login_id() function

Previously, many servers and services didn't support login by real name
(e.g. issue #469) even if the sysop had that option enabled in SCFG.

Move login control settings from node.ini to system (main.ini -> login)

The 3 node toggle options:
- Allow Login by User Number
- Allow Login by Real Name
- Always Prompt for Password

... have been now moved from SCFG->Nodes->Node x->Toggle Options to
SCFG-System->Toggle Options.

If you upgraded to v3.20a before now, you'll want to double-check these
settings to make sure they're how you want them set. New upgraders that run
upgrade_to_v320.js (e.g. via 'jsexec update') will get these settings migrated
automatically.

Added some error detection/logging to upgrade_to_v320.js when failing to open
.cnf files.

Constified some more user/login related function args and return types.