`pargs(1)` support without `/proc`

[basil]

If we don't have access to /proc (e.g., a core dump without systemd-coredump metadata), recall that the initial stack layout places argc, then argv pointers, then envp pointers. We should be able to find the stack pointer from NT_PRSTATUS or by locating AT_EXECFN and scanning backward.

Open question: Should we try to use program_invocation_name? Is that available in all C libraries?