Privilege letters/symbols should be set from isupport.PREFIX

[half-duplex]

The problem

If a server uses custom privilege symbols instead of something resembling our default (+vhoaqyY), at best, we will not know about them.

The solution

When available, set privilege letters/symbols from isupport.PREFIX. See https://modern.ircdocs.horse/#prefix-parameter

Notes

We'll need to think about how this will interact with things like our VOICE/OP/HALFOP/etc constants...