geom.8: Improve classes list into a table

Add missing entries MD, VFS, and DISK. Refactor list into a three
column table, so the data is all visible at once, buying us ten lines
at MANWIDTH 80, and still rendering nicely at MANWIDTH 59.

PR: 292530
MFC after: 3 days
Reported by: Slawomir Wojciech Wojtczak <vermaden@interia.pl>