Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/libc/libc_musl_linux_s390x.go

@@ -0,0 +1,50 @@
+// Copyright 2023 The Libc Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package libc // import "modernc.org/libc"
+
+import (
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+type long = int64
+
+type ulong = uint64
+
+// RawMem represents the biggest byte array the runtime can handle
+type RawMem [1<<50 - 1]byte
+
+func Xfesetround(tls *TLS, r int32) (r1 int32) {
+	if __ccgo_strace {
+		trc("tls=%v r=%v, (%v:)", tls, r, origin(2))
+		defer func() { trc("-> %v", r1) }()
+	}
+	return X__fesetround(tls, r)
+}
+
+func Xmmap(tls *TLS, start uintptr, len1 Tsize_t, prot int32, flags int32, fd int32, off Toff_t) (r uintptr) {
+	if __ccgo_strace {
+		trc("tls=%v start=%v len1=%v prot=%v flags=%v fd=%v off=%v, (%v:)", tls, start, len1, prot, flags, fd, off, origin(2))
+		defer func() { trc("-> %v", r) }()
+	}
+	return ___mmap(tls, start, len1, prot, flags, fd, off)
+}
+
+func ___mmap(tls *TLS, start uintptr, len1 Tsize_t, prot int32, flags int32, fd int32, off Toff_t) (r uintptr) {
+	if __ccgo_strace {
+		trc("tls=%v start=%v len1=%v prot=%v flags=%v fd=%v off=%v, (%v:)", tls, start, len1, prot, flags, fd, off, origin(2))
+		defer func() { trc("-> %v", r) }()
+	}
+	// https://github.com/golang/go/blob/7d822af4500831d131562f17dcf53374469d823e/src/syscall/syscall_linux_s390x.go#L77
+	args := [6]uintptr{start, uintptr(len1), uintptr(prot), uintptr(flags), uintptr(fd), uintptr(off)}
+	data, _, err := unix.Syscall(unix.SYS_MMAP, uintptr(unsafe.Pointer(&args[0])), 0, 0)
+	if err != 0 {
+		tls.setErrno(int32(err))
+		return ^uintptr(0) // (void*)-1
+	}
+
+	return data
+}