Pass arguments to main via runtime.process.Init, dispatched on signature

docs/sysv.md

@@ -90,8 +90,10 @@ process) instead of silently corrupting the image
 (`buildEntryStack` in `system/kernel/process.zig`); `argv[0]` is always the path
 or initial-ramdisk name the process was spawned as, and `system_spawn`'s optional
 argument blob becomes `argv[1..]`. The runtime's `_start`
-(`library/runtime/start.zig`) hands the block to `rt_start`, which exposes it as
-`runtime.argumentCount()` / `runtime.argument(i)`. A C runtime's `crt0` would walk
+(`library/runtime/start.zig`) hands the block to `rt_start`, which builds a
+`runtime.process.Init` from it and passes that to the program's `main`
+(`pub fn main(init: runtime.process.Init)`; a parameterless `main()` is also
+accepted). A C runtime's `crt0` would walk
 the identical layout unmodified — that's the compatibility being bought. The
 `args` test proves the round trip.