Add no_log_capture feature

[jsha]

Right now rustls-ffi makes the assumption that it's built by the Makefile, with -C metadata=rustls-ffi, and so it's safe to take over the global log facility for itself, because that facility is now scoped to the library rather than global (#107).

However, when we're used as an rlib (#41), that won't be the case, and we should specifically avoid trying to take over the global log facility. We should do this with a feature flag that inhibits the log capture. Code that uses rustls-ffi via rlib will be able to provide its own log handler if it wants to.