security: AT_EXECFN validation + audit.yml (#154, #155)

#154: Reject setuid multicall invocations where argv[0] doesn't match
AT_EXECFN from the ELF auxiliary vector. Prevents an attacker from
spoofing argv[0] to route the multicall binary to a different tool
in setuid context. Only enforced when euid != uid.

#155: Add daily cargo-audit workflow matching uutils/coreutils pattern.
Also triggers on Cargo.toml/Cargo.lock changes.

Fixes #154. Fixes #155.

.github/workflows/audit.yml

@@ -0,0 +1,19 @@
+name: Security audit
+
+# spell-checker:ignore (misc) rustsec
+
+on:
+  schedule:
+    - cron: "0 0 * * *"
+  push:
+    paths:
+      - "**/Cargo.toml"
+      - "**/Cargo.lock"
+jobs:
+  audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: rustsec/audit-check@v2
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}