tor-listeners.listeners was checked against listen,
but listen is obsolete now, so config load would fail
on a config that used the new style `listeners` instead of
`listen` --- unless tor-listeners.listeners was empty, which
it should be under the new style, but there's no need to enforce
that

1. DRY "banner" descriptions of each services
2. `/HELP NICKSERV` now refers the user to `/NICKSERV HELP`, etc.
3. Fix double translation of `textGenerator` output

1. Your own snomasks would be reported as belonging to the other user
2. Prefixes were incorrect

dan: "I probably wouldn't expect those mappings to cause much
trouble at all for legitimate users at the end of the day"

shivaram: [concurs]

'm' skeletonizes to 'rn' (but is exempted by the isBoring check),
but the fullwidth 'ｍ' does not skeletonize to anything. The root cause
of this is the (still unexplained) patchiness of the skeleton mapping
for fullwidth -> standard-width Latin characters; the fix is to perform
width mapping first, before either skeletonization or isBoring.

Partially fixed earlier in #467

now that we have client.destroyed again, we don't need to test the
"0 sessions && brb disabled" condition in both AddSession() and
destroy()

Use []uint32 in bitset instead of []uint64, because it's harder to guarantee
64-bit alignment of []uint64 than I had realized:

https://go101.org/article/memory-layout.html

Fixes #519

Quit messages aren't supposed to clobber each other under any
circumstances, but actually only the default "Connection closed"
message was non-clobbering, all others would clobber.

Also ensure that people can remove their own persistent modes via CS AMODE

Fixes #505