Implement Ethernet subsystem with unified NIC abstraction (AI)

- Created src/ethernet.h: eth_iface_t interface struct, Ethernet header
  type, htons/ntohs/htonl/ntohl byte-order helpers, EtherType constants,
  send/recv with automatic header construction/stripping
- Created src/ethernet.c: interface table, char device ops routed
  through ethernet layer, sysfs /sys/net namespace exposing per-iface
  mac/ip/netmask/gateway/link files, IPv4 address parse/format
- NE2000 and 3C509B drivers now register through ethernet_register()
  instead of directly via devicefs_register_char(); removed redundant
  char device ops from both drivers
- Kernel calls ethernet_init() before init_drivers() so the subsystem
  is ready when NIC drivers probe
- Tested: NE2000 detected with NIC, 'eth1' registered via ethernet
  subsystem; clean boot without NIC

README.md

@@ -70,7 +70,7 @@ Once a task is completed, it should be checked off.
 - [x] Create an app called `mkfs.fat32`. This app can be used to format a block into a FAT32 filesystem.
 - [x] Create a network driver for the NE2000 NIC.
 - [x] Create a network driver for the 3C509B NIC. It should only support RJ45 and 10base-T.
-- [ ] Create an ethernet subsytsem. Each ethernet device should be shown as a character device with the name `ethN`.
+- [x] Create an ethernet subsytsem. Each ethernet device should be shown as a character device with the name `ethN`.
 - [ ] Create a IPv4 stack. Create the `ip` app that shows curernt IPv4 configuration. It should read this information from `/sys`
 - [ ] Create a ARP subsystem. Create the `arp` command that shows current ARP tables. Again, this info should be found in `/sys`
 - [ ] Create a DHCP subsystem. Create the `dhcp` command to show current DHCP status information.