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There's really no reason to *only* do this if we have relays enabled. Doing it
this way makes `go vet` happier and reduces conditional logic.

The legacy interface stays unchanged with a panic though.

There were previously 4 different ways of passing various options
into BasicHost construction.

1. Function parameters to New
2. Option parameters to New
3. Global variables, e.g. NegotiateTimeout
4. Options struct, in the calling code in go-ipfs

This changeset deprecated all of these, and introduces the HostOpts
struct to replace them. It also introduces a new constructor called
NewHost, which accepts a *HostOpts.

The old New constructor continues to work the same way,
but is from now on deprecated too.