m/n/core: run dedicated PID 1 reaper

This introduces minit, a tiny init implementation, written in C, built
against musl. It does one thing: reap children. No support for TTY, no
configurability, just the bare minimum for a working system.

We also drive-by remove some dead code from main.go.

This solves https://github.com/monogon-dev/monogon/issues/15

Change-Id: I666ff2042f19639465ff918590a39b8e219ee7d6
Reviewed-on: https://review.monogon.dev/c/monogon/+/346
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/node/core/minit/BUILD.bazel b/metropolis/node/core/minit/BUILD.bazel
new file mode 100644
index 0000000..573ca92
--- /dev/null
+++ b/metropolis/node/core/minit/BUILD.bazel
@@ -0,0 +1,9 @@
+cc_binary(
+    name = "minit",
+    srcs = [
+        "main.c",
+    ],
+    visibility = [
+        "//metropolis/node:__pkg__",
+    ],
+)