netifd: expose udhcpc timeout/retry/tryagain UCI options in dhcp.sh

netifd: expose udhcpc timeout/retry/tryagain UCI options in dhcp.sh

proto/dhcp.sh hardcoded `-t 0` (infinite retries) with no way
for UCI / operator to tune per-retry interval (udhcpc -T),
retry count (udhcpc -t) or failure retry-loop wait (udhcpc -A).
Add three new proto config ints (timeout, retry, tryagain)
and thread them into the udhcpc command line. Unset options
preserve existing behaviour: retry defaults to 0, timeout and
tryagain are omitted so udhcpc uses its own RFC defaults.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>