loader: create separate man pages for each of the loaders

[imp]

Summary

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

Test Plan

Note: this is purely a re-org commit, plus a sentence or two for pointers, etc.
A more extensive rework of lua likely is warranted.
As is a full audit of all the built-in commands and env variables.

[imp]

stand/man/loader.8

3

I'm less sure this belongs here and maybe it will belong elsewhere in the future.

[gbe]

Just one small nit.

stand/man/loader.8

66

The .Lk macro should be used for URLs.

[juan.molina_club.fr]

stand/man/loader.8

65

s/is includes/includes/