net/dhcp6: several fixes

• rcorder incorrect (Circular dependency on provision `DAEMON')
• command_flags do not need to be referenced inside of the command_args
• dhcp6s exits because of shutdown on non-connected UDP socket

PR: 216673, 217452, 228994
Submitted by: tdb, Andrey Pevnev <apevnev@me.com>, leres, Dries Michiels <driesm.michiels@gmail.com>
Approved by: hrs (maintainer timeout)
Differential Revision: D16994