More convenient API for installing global FD list

[trombonehero]

In order to install a global FD list, the client application must currently:

1. create an anonymous shared memory segment,
2. copy an FD list into the shared memory, and
3. set the segment's descriptor as the global list.

libcapsicum should just handle all of this (possibly with one call for 1+2 and another for 3, as well as a wrapper around it all).