Add smbios sysctl entries for fields for SMBIOS up to version 2.4

[stevek]

Summary

Add sysctls under smbios.system for the following SMBIOS Type 1 fields:
smbios.system.sku - SKU Number (SMBIOS 2.4 and above)
smbios.system.family - Family (SMBIOS 2.4 and above)

Add sysctls under smbios.planar for the following SMBIOS Type 2 fields:
smbios.planar.tag - Asset Tag
smbios.planar.location - Location in Chassis

Sponsored by Juniper Networks, Inc.

Test Plan

Same changes used in Juniper builds of the FreeBSD kernel for some platforms.
On the platforms that have SMBIOS 2.4 or above, the sysctls get populated
with the correct values. (The planar sysctls are populated regardless of
the version of SMBIOS and have been verified.)

[jhb]

I would just say that these are kernel environment variables rather than sysctls in the commit message.

[stevek]

In D7453#155148, @jhb wrote:

I would just say that these are kernel environment variables rather than sysctls in the commit message.

Good point, I will adjust that. I think my head has been in sysctls way too much lately that my fingers decided to type sysctl when I wrote that.