rc.shutdown: print a newline before watchdog timeout message

Previously the watchdog timeout message was appended to the last entry
in the "Waiting for PIDS" list, resulting in a message like

Waiting for PIDS: 31299 31296 90201 9020090 second watchdog timeout
expired. Shutdown terminated.

Print a newline to separate the watchdog timeout message. Also perform
the kill before logging or echoing the message.

PR: 241072
MFC after: 1 month
Sponsored by: The FreeBSD Foundation