`syscall(SYS_pidfd_open)` should produce clear error when `ENOSYS`

While building on an older Linux kernel for compatibility, all V2 process spawns failed with `Bad file descriptor`. We traced this back to `syscall(SYS_pidfd_open)` returning `-1` and `errno == ENOSYS`.

In config.hpp, even though SYS_pidfd_open is #defined, it doesn't appear to mean that it will work.

We now detect and define `BOOST_PROCESS_V2_DISABLE_PIDFD_OPEN` and things work as expected, but it took some time to figure out that we needed to do this.

It would be nice if an exception was thrown when this failed syscall produced ENOSYS.