Pass argv to processes on a SysV entry stack; grow the user stack to 32 KiB

Processes now start with C-compatible arguments: the kernel builds the
System V AMD64 entry block (argc, argv, empty envp, auxiliary vector)
at the top of the stack, argv[0] is the path or initial-ramdisk name
the process was spawned as, and system_spawn carries an optional
NUL-separated blob that becomes argv[1..]. The runtime parses the block
(runtime.argumentCount/argument) and its spawn wrappers pass arguments
through. The name is also recorded on the task, so a fault report says
which binary died, not just its id.

The user stack grows from one page to eight (32 KiB,
parameters.user_stack_pages), with the page below left unmapped as a
guard so an overflow faults into a clean process kill rather than
corrupting the image. Task.name_buffer is zero-initialised, not
undefined: an undefined default is materialised as a 0xAA fill that
moved the static task pool out of .bss and made the whole kernel ~7x
slower under QEMU TCG (caught by the affinity test).

Proven end to end by the new args test: args-echo respawns itself with
arguments via the syscall blob, burns more stack than one page could
hold, and echoes its argv intact. Full suite: 44/44.

test/qemu_test.py

@@ -205,6 +205,11 @@ CASES = [
      "timeout": 60,
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Process arguments: argv arrives on the SysV entry stack (argv[0] = the spawned
+    # name, argv[1..] = the system_spawn argument blob) and echoes back intact.
+    {"name": "args",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The real user binary: the bootloader ships /system/services/init off the ESP, the
     # kernel loads the ELF and runs it in ring 3, and it writes + exits cleanly.
     {"name": "init",