loader: create separate man pages for each of the loaders

Create a man page per loader. Loader(8) will have information common to
all of them, while loader_${INTERP}(8) will have information relevant to
that specific loader. Rewrite loader(8) to give an overview and point to
the appropriate man page. Rewrite each of the loader_${INTER}(8) man
pages to contain only the relevant information to that loader. Put all
the common commands, environment variables, etc in loader_simp(8) and
refernce that from the loader_lua or loader_4th man pages. The
loader_lua(8) could use more details about the Lua
integration. Additional organization may be benefitial.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31340