Support per-sub-board user signature (.sig files)

This feature was long overdue.

And "SIG" command to view, create or edit default signature (same function as
available from user_settings.js, in fact a copy/pasted the code).
Add "SUBSIG" command to view, create, or edit sub-board signture (no other
method is available to this other than direct file system access, e.g. by the
sysop).