libargs

On most Rust platforms, std::env::args will work in a function called from a C program. However, this is not the case on Linux. The following platforms are supported:

• cfg(all(target_os = "linux", target_env = "gnu")): glibc will pass command line arguments to static constructors, which is non-standard
• cfg(all(target_os = "linux", not(target_env = "gnu"))): The envp pointer passed to _start is stored in the __environ symbol. As command line arguments precede it in the stack, we can walk the stack backwards from __environ to find argc and argv. This works on all platforms I've tested it on, however it takes linear time with regards to argc.
• cfg(not(target_os = "linux")): std::env::args is used, which works on most platforms (including Windows and macOS). On unsupported platforms, an empty Vec will be returned.