Generalise hardware address printing

Generalise hardware address printing

* defs.h [!MAX_ADDR_LEN] (MAX_ADDR_LEN): New macro.
(sprint_hwaddr): New declaration.
(print_hwaddr): New inline function, a wrapper for sprint_hwaddr.
* print_fields.h (PRINT_FIELD_HWADDR_SZ): New macro.
* print_mac.c: Include "xlat/arp_hardware_types.h" under
XLAT_MACROS_ONLY.
[!MAX_ADDR_LEN] (MAX_ADDR_LEN): Remove.
(sprint_hwaddr): New function.
* sock.c (print_ifreq) <SIOCSIFHWADDR, SIOCGIFHWADDR>: Print hardware
address using PRINT_FIELD_HWADDR_SZ.