fix(dpdk): prepend program-name placeholder in eal::init

rte_eal_init treats argv[0] as the program name and skips it during
option parsing.  The wrapper was passing the caller's args verbatim,
so the first flag was silently swallowed on every call -- including
in production callers and in any test that hands eal::init a list of
flags.

Fix by prepending a static `c"dataplane"` placeholder inside the
wrapper.  The placeholder lives in .rodata; DPDK's argparse only
mutates the argv pointer array, never the strings, so the cast_mut
is sound (documented as a SAFETY comment).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: daniel-noland

dpdk/src/eal.rs

@@ -124,10 +124,21 @@ pub fn init(args: impl IntoIterator<Item = impl AsRef<str>>) -> Eal {
     // The easiest way I know how to do that is by bundling the pre-shift logic into its own scope.
     // The system memory will be free by the time this scope closes.
     let eal = {
+        const ARG0: &CStr = c"dataplane";
         let mut args = ValidatedEalArgs::new(args).unwrap_or_else(|e| {
             Eal::fatal_error(e.to_string());
         });
-        let mut c_args: Vec<_> = args.0.iter_mut().map(|s| s.as_ptr().cast_mut()).collect();
+        // EAL treats argv[0] as the program name and ignores it; this
+        // slot would otherwise eat the first real flag.
+        // We side step this by prepending a dummy program name pointer.
+        // SAFETY: ARG0 points into .rodata; DPDK reads argv[0] for prog_name and
+        // only mutates the argv array's pointer slots, never the strings.  The
+        // cast_mut is a type accommodation for the FFI signature, not a license
+        // to write through the pointer.
+        let mut c_args: Vec<_> = [ARG0.as_ptr().cast_mut()]
+            .into_iter()
+            .chain(args.0.iter_mut().map(|s| s.as_ptr().cast_mut()))
+            .collect();
         let ret = unsafe { dpdk_sys::rte_eal_init(c_args.len() as _, c_args.as_mut_ptr() as _) };
         if ret < 0 {
             EalErrno::assert(unsafe { dpdk_sys::rte_errno_get() });