File descriptor leak check.

This mod

val run_check_at_exit : bool ref
Toggle to turn on/off checking for descriptor leaks at exit (default: off)
val critical : float ref
Fraction of maximum number of descriptors considered critical. Default: 0.9
val report_open_files : unit -> unit
report_open_files () prints a dump of open file descriptors to stderr in two formats, one using the proc file system, the other by executing /usr/sbin/lsof in a child process.
val report_on_exn : exn -> unit
report_on_exn exn calls report_open_files iff exn indicates a file descriptor leak (Unix error with code EMFILE or ENFILE).
val get_num_open_fds : unit -> int
val percent_fds_in_use : unit -> float
percent_fds_in_use () reports the percentage of fds that are in use.