bsdinstall: Suggest the GPT+Active workaround on Dell T5810

The Dell Precision Tower 5810 fails to boot from GPT in Legacy/BIOS mode
without the Active flag in the Protective MBR. Suggest the workaround
during installation.

Since an increasing number of Dell systems exhibit this behavior,
I imagine all Dells past a certain date will do so. I would like
to suggest the workaround for all Dells with a BIOS date of, say,
2014 or later, but I would need to test a variety of systems before
committing such a change.

Reviewed by: allanjude, dteske
MFC after: 5 days
Relnotes: We should probably suggest using GPT+Active on "recent" Dells.
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D4075