testers needed: loader: use display pixel density for font autoselection

hi!

I have done some work to make font pickup a bit smarter (hopefully better;), but my own ability to test is limited to one bugged supermicro and one MBP with retina display… 

The phab link is https://reviews.freebsd.org/D28849 <https://reviews.freebsd.org/D28849>

I have built loader binaries as well (bios and uefi):
loader_lua <http://148-52-235-80.sta.estpak.ee/loader_lua>
loader_lua.efi <http://148-52-235-80.sta.estpak.ee/loader_lua.efi>

To test, you should remove screen.font= line from loader.conf and test with different resolutions.

thanks,
toomas