Use C99 format macro constants for timestamp and vlan_tag

[jefferyto]

Since timestamp and vlan_tag in the shm_log_entry struct are C99 fixed width integer types (uint64_t and uint16_t), the cross-platform way to print these values is to use the corresponding format macro constants, PRIu64 and PRIu16.

This also adjusts the places where the time_t timestamp value is printed, casting it to uint64_t, for consistency.

Fixes https://github.com/fln/addrwatch/issues/25 Fixes https://github.com/fln/addrwatch/issues/26