encode.h and str.h are not used in any open PRs and can safely be renamed
with a coap_ prefix.
Order the include lists alphabetically as if all the entries have a coap_
prefix, so when the include file gets renamed with a coap_ prefix, the
include order is not disturbed.
Rename (as these are all internal files) for clarity
coap_mutex.h to coap_mutex_internal.h
coap_hashkey.h to coap_hashkey_internal.h
uthash.h to coap_uthash_internal.h
utlist.h to coap_utlist_internal.h