nixos/dnscrypt-proxy: replace unimportant options with extraArgs

Removes tcpOnly and ephemeralKeys: reifying them as nixos
options adds little beyond improved discoverability.  Until
17.09 we'll automatically translate these options into extraArgs
for convenience.

Unless reifying an option is necessary for conditional
computation or greatly simplifies configuration/reduces risk of
misconfiguration, it should go into extraArgs instead.