Introduce ntp_manage_config interface
Fix bug 529204 - Support a dhcpc_script_t domain

We introduce an executable domain (dhcpc_script_t) through which the
hooks can be executed for the DHCP clients. This domain is separate in
order to keep the privileges of the application small, but also because
this domain will execute commands that are not in the responsibility of
the DHCP client code itself (code-wise) but is provided by
administrators.

Security-wise, as these are scripts, it is more difficult to guarantee
correctness. As such, we want to isolate these privileges into its own
domain.

The domain will have basic privileges to support the majority of
installations, but we also include a sysnet_dhcpc_script_entry()
interface so that domain transitions can be easily added without the
need for augmenting the privileges of the dhcpc_script_t domain.