lualoader: Implement try_include and use it for including the local module

This provides a way to optionally include a module without having to wrap it
in filesystem checks. try_include is a little more robust, using the lua
search path instead of forcing us to explicitly consider all of the places
we could want to include a module. Errors are still generally raised from
trying to load the module, but ENOENT will not get raised unless we're doing
a verbose load.

This will also be used to split out logo/brand graphics into their own files
so that we can safely scale up the number of graphics included without
worrying about the extra memory consumption- opting to lazily load graphics
instead.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D14658