Unbreak "last reboot".

According to the last(1) man page, the "reboot" pseudo-user should print
all system reboot entries. This got broken by the utmpx import, as
records are typed.

Re-add support for "last reboot" by specifically matching against
SHUTDOWN_TIME and BOOT_TIME records.

PR: 168844
Submitted by: matthew@
MFC after: 1 month