dev: read /dev/zero and /dev/urandom from them, not from the keyboard

consoleinit() put consoleread/consolewrite in devsw[1], over memread and
memwrite. Major 1 holds the whole mem major (null, zero, full, random,
urandom) and memread already sends the console's minors to the console,
so the override only broke the others: a read of /dev/zero waited for a
typed line. dd and head on either device hung, and `head -c 10 /dev/zero`
only finished because it swallowed the next command.

The console now installs just its open and close hooks. /dev/keyboard
writes are discarded, as consolewrite used to do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/dev/cdev.c

@@ -170,6 +170,7 @@ memwrite(struct inode *ip, uint64 off, char *dst, int n)
     case 10: case 11: case 12: case 13: case 14: case 15: case 16:
             return consolewrite(ip, off, dst, n);
     case 7: return fullwrite(ip, off, dst, n);   // /dev/full: always ENOSPC
+    case 3:                                      // /dev/keyboard: discarded
     case 2:
     case 5:
     case 8: