Actually set tls.Config.PreferServerCipherSuites
Actually set tls.Config.PreferServerCipherSuites

It was set by default on the caddy-internal config object, and even
checked for conflicts, but it was never actually reflected on the
tls.Config.

This will have user-visible changes: a client that prefers, say, AES-CBC
but also supports AES-GCM would have used AES-CBC befor this, and will
use AES-GCM after.

This is desirable and important behavior, because if for example the
server wanted to support 3DES, but *only if it was strictly necessary*,
it would have had no way of doing so with PreferServerCipherSuites
false, as the client preference would have won.

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