Add support for bind mounts using mount(2)

Assuming a the directory "/tmp/mnt" exists:

    1> Port = alcove_drv:start([{exec, "sudo"}]).

    % Fork a process with a new mount namespace
    2> {ok, Child} = alcove:clone(Port, alcove:clone_define(Port, newns)).

    % Read-only bind mount of /bin on /tmp/mnt
    3> alcove:mount(Port, [Child], "/bin", "/tmp/mnt", "", 4096 bxor 1, "").

    % Run "/bin/ls -al /tmp/mnt" in the mount namespace
    4> alcove:execvp(Port, [Child], "/bin/ls", ["/bin/ls", "-al", "/tmp/mnt]).
Add umount(2) and mount_define/2
tests: mount(2)/umount(2)