Clock divisors 0-3 correspond to dividing by 1-4, so add 1 before dividing.
Add support for "first boot" rc.d scripts. [1]

These scripts, containing
# KEYWORD: firstboot
will only be run if a sentinel file (default: /firstboot, configurable
via the rc.conf ${firstboot_sentinel} variable) exists; this sentinel
file will be deleted at the end of the boot process.

Scripts can request that the system reboot after the first boot by
creating the file ${firstboot_sentinel}-reboot.

This functionality is expected to be useful for embedded systems and
virtual machine images, where it may be desirable to
(a) download and install updates which became available between when
the image was created and when it was "turned on";
(b) download and install packages which may be newer than those
which were available when the image was created;
(c) install packages which run binaries during their install process,
bypassing the problem of cross-architecture installs;
(d) resize filesystems to match the disk onto which a VM image was
installed;
(e) perform initialization tasks relevant to cloud systems (e.g.,
Amazon's Elastic Compute Cloud);
and likely to perform many other one-time initialization functions.

Document this new functionality in rc.conf(5) and rc(8). [2]

Reviewed by:	freebsd-current, freebsd-rc [1]
Reviewed by:	Warren Block [2]
MFC after:	3 days
Bump __FreeBSD_version to 1100001 for addition of support for "first boot"
rc.d scripts, so that ports can make use of this.