lualoader: add loader_conf_dirs support (loader.conf.d)

loader_conf_dirs is the supporting mechanism for the included
/boot/loader.conf.d directory. When lualoader finishes processing all of
the loader_conf_files it finds after walking /boot/defaults/loader.conf,
it will now check any and all loader_conf_dirs and process files ending
in ".conf" as if they were a loader.conf.

Note that loader_conf_files may be specified in a loader.conf.d config
file, but loader_conf_dirs may *not*. It will only be processed as specified
in /boot/defaults/loader.conf and any loader_conf_files that were loaded
from there.

Reviewed by: allanjude, freqlabs, rpokala, tsoome
Includes suggestion from: imp
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D25608