Cross-platform polyfills.
This library attempts to provide reliable pollyfills for functionality that isn't implemented on all platforms, for example `gettid`, `memfd_create`, `fexecve`, as well as providing non-atomic versions of functions like `accept4`, `socket`+`SOCK_CLOEXEC`, `pipe2`, an other miscellanea like `seal` to make a file descriptor read-only thus suitable for `fexecve`.