Refuse to act if argv[1] contains spaces

A possibility for future extension is to tokenise `argv[1]` as a command line ourselves. This is because, on Linux, only one argument is passed from the shebang line - effectively the shebang line is split once at the first space. Relexec requires an argument and thus "consumes" the available argument.

This behaviour is platform dependent, and the exact semantics of this are debatable, but to preserve the flexibility to pursue this direction, we could simply error out if the relative path argument (`argv[1]`) contains spaces. This would prevent users depending on a specific interpretation of spaces.

Start by locating the Python entry point that reads argv[1] and inspect how invalid arguments are currently reported. Verify the behavior for a relative path containing spaces on the supported platforms; done means the program refuses that input without changing handling for arguments without spaces.

Written by the indexing model from the issue text.