loopd, bxctl, inferproxy: read args_os instead of panicking

M3a review finding 7, for the other three roles. loopd keeps its config path
as a path; bxctl and inferproxy take text arguments and answer one that is
not UTF-8 with their usage.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/inferproxy/tests/args_os.rs

@@ -0,0 +1,14 @@
+//! An argument that is not UTF-8 is a usage error, not a panic (M3a review finding 7).
+
+use std::ffi::OsStr;
+use std::os::unix::ffi::OsStrExt;
+use std::process::Command;
+
+#[test]
+fn an_argument_that_is_not_utf8_is_a_usage_error() {
+    let out = Command::new(env!("CARGO_BIN_EXE_inferproxy"))
+        .args([OsStr::new("--listen"), OsStr::from_bytes(b"/tmp/\xff.sock")])
+        .output()
+        .unwrap();
+    assert_eq!(out.status.code(), Some(2));
+}