This requires execute_buffer() and its callers to distinguish between
fatal and non-fatal errors. The -99 return value was already used for
non-recoverable errors that should force the daemon to restart, but the
execute_buffer() callers treated any non-success return as a fatal
error.
There a judgement call here on how to handle various error types. I
choose to assume that failures to set the IP address, netmask, broadcast
address, or default router are fatal errors. ndhc should be run from
process supervision, and this will trigger a daemon restart, which will
allow the machine to recover as soon as the problem (probably on the
dhcp server or local kernel state outside of ndhc's control) is
corrected.
This change corrects errors such as:
Discovering DHCP servers...
(process_client_socket) received invalid commands: 'carrier:;'
(send_dhcp_raw) carrier down; sendto would fail
Failed to send a discover request packet.
which happened if ndhc is started on a machine where the network
interface is down. After this change, ndhc should function as intended
by going to sleep until the carrier returns rather than terminating
itself to be restarted by the process supervisor until carrier returns.
