core/topic: Check CMODE_TOPIC in ->mode, not ->flags
Add WHO
msg: Properly unload commands
Add MOD{,UN,RE}LOAD and MODLIST commands (see full commit msg)

MODLIST in Tethys TS6 is a hunted command, as opposed to Charybdis TS6
where it is a local-only command. This change is not expected to conflict
with any TS6 software, as any server receiving a remote MODLIST should
be robust enough to handle it in a useful manner.