cmd: Fix bug preventing --lightserv NN w/ --syncmode=full

[ryanschneider]

Currently in master this combination of flags fails:

$ ./build/bin/geth dumpconfig --lightserv 90 --syncmode=full --gcmode=archive
INFO [10-01|14:15:20.621] Maximum peer count                       ETH=25 LES=100 total=125
Fatal: Flags --lightserv, --syncmode can't be used at the same time

However, the intent was to prevent --lightserv NN and --syncmode=light.
There was a bug in the validation logic, this PR fixes said logic so that the above works, but the intended conflict still fails:

$ ./build/bin/geth dumpconfig --lightserv 90 --syncmode=light
INFO [10-01|14:18:05.744] Maximum peer count                       ETH=0 LES=100 total=125
Fatal: Flags --lightserv, --syncmode=light can't be used at the same time

$ ./build/bin/geth dumpconfig --lightserv 90 --syncmode=full
INFO [10-01|14:18:22.200] Maximum peer count                       ETH=25 LES=100 total=125
[Eth]
NetworkId = 1
...rest of config elided..

Note this is effectively a refresh of #16862 which was sitting in PR limbo for awhile.

[ryanschneider]

FWIW the Appveyor build failures seem to be unrelated failures w/ swarm:

# github.com/ethereum/go-ethereum/cmd/swarm [github.com/ethereum/go-ethereum/cmd/swarm.test]
378cmd\swarm\run_test.go:245:30: undefined: loglevel
379cmd\swarm\run_test.go:321:30: undefined: loglevel
380util.go:45: exit status 2
381exit status 1
382Command exited with code 1

Also FWIW I don't run into these failures when running the tests locally:

$ GOCACHE=off go test ./cmd/utils/...
ok  	github.com/ethereum/go-ethereum/cmd/utils	0.048s

[ryanschneider]

Based off past interactions I think @fjl or @holiman would both be good candidates as reviewers for this PR, sorry for the pings but this module isn't listed in CODEOWNERS.

[ryanschneider]

@karalabe would be nice to get this in 1.8.17, as it lets Archive nodes act as LES servers, whereas currently they can't due to this parsing bug.

[karalabe]

Huh, weird corner case and good catch. Took me a long while to understand what's wrong and why your patch fixes it. LGTM!