libc_service_cat.lua

This commit touches many different parts of the project:

* It introduces a new optional library to override functions from libc
  that relate to ambient authority.
* If this new library is linked with the binary, libemilua will make use
  of its features to offer its own replacement functions for a few libc
  functions.
* It becomes possible to spawn actors where the caller can opt to act as
  a policy manager for libc calls in the spawned actor. This allows one
  to fake the implementation of some libc functions that don't work in
  capsicum mode.
* Yet another library is introduced so it becomes to do the same for any binary
  in the system using `LD_PRELOAD`.

Fixes #158