Report an unparseable shim command line instead of panicking

[arpitjain099]

Fixes #1180.

shim_main unwraps the result of parse, so any flag the shim does not define, --version included, aborts with a panic and exit 101 instead of saying what was wrong. argv0 moves above the parse so the message can name the binary, and the error goes to stderr with exit 1.

I could not build a shim binary on macOS (procfs is Linux only), so I reproduced the two behaviours against the same containerd_shim::parse in a Linux container, one binary per shape:

--- before, unknown flag:
thread 'main' panicked at src/bin/before.rs:6:38:
called `Result::unwrap()` on an `Err` value: InvalidArgument("flag provided but not defined: -version")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
exit=101

--- after, unknown flag:
after: Failed to parse command line: flag provided but not defined: -version
exit=1

--- after, -v:
after: parsed, version flag = true
exit=0

cargo check -p containerd-shimkit and cargo fmt -p containerd-shimkit -- --check are clean, both run in a rust:1.91-slim container to match the pinned toolchain.

There is no unit test here because the branch ends in std::process::exit, which a test in this crate cannot observe. If you would rather have coverage, the parse and the exit could be split so the decision is testable, but that is a bigger change than the bug needs, so I left it.