odhcp6c: prevent RELEASE at shutdown when -k is set

odhcp6c: prevent RELEASE at shutdown when -k is set

Problem: Based on the README, if -k option is set, a RELEASE message
should not be sent when the process is stopped. RELEASE messages are
also not sent in case of a forced RELEASE or reconfiguration, however.

Solution: Stop the transmission of RELEASE messages when the process is
stopped but allow it for a forced RELEASE or reconfiguration.

Signed-off-by: Nicolas BESNARD <[email protected]>
Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <[email protected]>