cmd/swarm, swarm/api: bzzr improve + networkid prio

fixes #3444
fixes #3494
networkid override

Added comments to explain why test against 0 appears twice

* Command line overrides saved config, saved config overrides system default

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fixes #3476
bzzr get with path

Finally a hopefully clean commit for this PR
Added check for empty path to avoid SIGSEGV in path parser and resolver
Added requested tests for empty path and non-existing manifest.
However signature for StartHTTPServer had changed.
Now it's hacked as so:

	StartHttpServer(api.API, &Server{Addr: "127.0.0.1:8504", CorsString: ""})

* Parse url before resolve when path and ENS is supplied, example
* swarm/api/http proxy server test for retrieval of subpath through get
* Removed nil entry assignment on subtrie leaf in recursive key retrieval
* Cleaned up path-or-no-path condition in proxy server get handler
* swarm: processed with gofmt refers to lash/go-ethereum@90daa7a
* swarm: Added public access method Parse alias to parse
* swarm: processed with gofmt References nolash/go-ethereum@2ec3fd7
* Rename parse to Parse, removed alias