Add support for Linux namespaces using clone(2)

Ugly, ugly code yet it compiles and the tests pass. I declare success!

To start a port in a new namespace:

    % see clone(2) for CLONE_NEW* flags
    1> Port = alcove_drv:start([
            {ns, "ipc"},
            {ns, "net"},
            {ns, "uts"},
            {ns, "ns"},
            {ns, "pid"}
            ]).

    2> alcove:execvp(Port, "/sbin/ifconfig", ["/sbin/ifconfig", "-a"]).
    ok

    3> IF = alcove:stdout(Port).
    <<"lo        Link encap:Local Loopback  \n          LOOPBACK MTU:16436  Metric:1\n          RX packets:0 errors:0 droppe"...>>

TODO:
* figure out how to test this
* add  ifdef'ed support for user namespaces
* test fork() still works
Add getpid(2)

Create a test that runs the port process in a new PID namespace and
tests that the child process is PID 1.