Implement 3C509B (EtherLink III) ISA Ethernet NIC driver (AI)

- Created src/e3c509.h: full windowed register model (8 windows),
  command codes, status/interrupt bits, RX filter, transceiver types,
  device struct and API declarations
- Created src/e3c509.c: full driver implementation with PIO TX/RX,
  window selection, 10base-T (RJ45) transceiver config, MAC address
  read from Window 2, FIFO-based packet send/receive, IRQ handler,
  devicefs char device registration as 'eth' class
- Probe uses manufacturer ID check (0x6D50 at Window 0)
- Only 10base-T supported per design requirements
- Wired IRQ 10 (vector 42) handler into isr.c
- QEMU does not emulate 3C509 ISA, so driver correctly probes
  'not found' in QEMU; tested alongside NE2000 without issues

README.md

@@ -68,8 +68,8 @@ Once a task is completed, it should be checked off.
 - [x] Add support for character device to the devicefs subsystem.
 - [x] Create an app called `diskpart`. This app can be used to modify the MBR partitions on a block device.
 - [x] Create an app called `mkfs.fat32`. This app can be used to format a block into a FAT32 filesystem.
-- [ ] Create a network driver for the NE2000 NIC.
-- [ ] Create a network driver for the 3C509B NIC. It should only support RJ45 and 10base-T.
+- [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`.
 - [ ] 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`