This simplifies (written and compiled) code by doing a head rather than
a tail insertion of the new listener into gcoap's list.
As handling of listeners without a link_encoder is now fixed,
gcoap_get_resource_list can handles this now without having to manually
skip over the .well-known/core handler (which is not the first entry any
more now).
Incidentally, this allows the user to install a custom handler for
.well-known/core, as the default handler is now evaluated last.
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