stand: lua: enhance lfs.dir() to speed up kernels_autodetect

This eliminates a lot of stat() calls that happen when lualoader renders the
menu with the default settings, and greatly speeds up rendering on my
laptop.

ftype is nil if loader/loader.efi hasn't been updated yet, falling back to
lfs.attributes() to test.

This is technically incompatible with lfs, but not in a particularly
terrible way.

Reviewed-by: cem
MFC-after: 4 days
Differential Revision: https://reviews.freebsd.org/D27542