Unix fcntl.h types, maps, and support

unix-fcntl provides access to the features exposed in fcntl.h in a way that is not tied to the implementation on the host system.

The Fcntl module provides functions for translating between the names of the flags exposed in fcntl.h and their values on particular systems. The Fcntl_host module exports representations of various hosts.

The Fcntl_unix provides bindings to functions that use the flags in Fcntl along with a representation of the host system. The bindings support a more comprehensive range of flags than the corresponding functions in the standard OCaml Unix module. The Fcntl_unix_lwt module exports non-blocking versions of the functions in Fcntl_unix based on the Lwt cooperative threading library.