Fix performance bug on hosts with large

Fix performance bug on hosts with large
/var/adm/wtmp files with no boot records.

(boot_time_initialized): New var.
(init_filelock): Initialize it.
(get_boot_time): Use it, instead of nonzero boot_time,
to test whether boot_time is initialized.
Don't invoke utmp routines when dumping.

(get_boot_time): First, try to get the boot time from the
current utmp file, as this can be much faster.  Don't try to
get the boot time from utmp and/or wtmp more than once.

(get_boot_time_1): When passed null pointer, don't invoke utmpname.
New arg LATEST.  All uses changed.