minor, allow more than 256U bytes payload by occupying the higher 4bits of the type

This means UDPC_TYPE_DATA (== 0x0cU) + 0x00 will be used for payloads <256 bytes
UDPC_TYPE_DATA + 0x10 for [256-512)
UDPC_TYPE_DATA + 0x20 for [512-768)
UDPC_TYPE_DATA + 0x30 for [768-1024)
...

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, unpack extended sizes in ud_chck_msg()

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
cosmetics, turn to uint8_t ptrs in ud_pack_cmsg(), ...

don't go as far though as to allow control messages >=256U payload.

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
hygiene, migrate to stdint.h's uint8_t for payload data in private structs

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
hygiene, mark unsigned operations with U suffix and use restricted pointers

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'core/msg' into next

* core/msg:
  hygiene, mark unsigned operations with U suffix and use restricted pointers
  hygiene, migrate to stdint.h's uint8_t for payload data in private structs
  cosmetics, turn to uint8_t ptrs in ud_pack_cmsg(), ...
  minor, unpack extended sizes in ud_chck_msg()
  minor, allow more than 256U bytes payload by occupying the higher 4bits of the type