Fix fd leak in RingBuffer; add fd tracking to find leaks (default off; opt-in).

[tonyastolfi]

This PR fixes a file descriptor (fd) leak in RingBuffer::Impl. It also adds some diagnostic tools to try to make it easier to find and diagnose future fd leaks.

• System open should only be called from llfs::system_open3 and llfs::system_open2
• All other syscalls that allocate and return file descriptors (e.g. eventfd and memfd) should be wrapped in llfs::maybe_track_fd
• When fd tracking is enabled, any call to maybe_track_fd will add a stack trace associated with the passed fd to a global table
• The llfs/track_fds.hpp header provides the following API for debugging fd leaks:
  • get_open_fds() returns a set of the fds currently open by the calling process
  • is_fd_tracking_enabled() and set_fd_tracking_enabled(bool) allow querying and modifying the tracker status
  • get_trace_for_fd(int fd) will return the last stack trace associated with the passed fd, if there is one
  • set_trace_for_fd(int fd, stacktrace) allows a stack trace to be manually associated with a given fd

I also took the opportunity to clean up the code in IoRing::File, eliminating redundancy by calling existing functions.

[gabrielbornstein]

Overall the changes look very clean and reduce code in some very nice ways! A couple questions, a couple comments and suggestions.