net/rdcli: new module structure and high-level doc

sys/Makefile

sys/net/application_layer/cord/common/Makefile

sys/net/application_layer/cord/common/rdcli_common.c

sys/net/application_layer/cord/doc.txt

sys/net/application_layer/cord/ep/Makefile

sys/net/application_layer/cord/ep/rdcli.c

sys/net/application_layer/cord/ep/rdcli_standalone.c

sys/net/application_layer/cord/epsim/Makefile

sys/net/application_layer/cord/epsim/rdcli_simple.c

...et/application_layer/cord/epsim/rdcli_simple_standalone.c