flua: support our flua modules in the bootstrap flua

This version builds every module into the flua binary itself, since all
of the bootstrap tools are built -DNO_SHARED. As a result, we also
canno

flua: support our flua modules in the bootstrap flua

This version builds every module into the flua binary itself, since all
of the bootstrap tools are built -DNO_SHARED. As a result, we also
cannot dlsym(), so we can't really discover the names of our newly
builtin modules. Instead, just build out a linker set with all of our
luaopen_*() functions to register everything up-front.

Building in all of the modules isn't strictly necessary, but it means
that we have an example of how to add a bootstrap module everywhere you
go and one doesn't need to consider whether bootstrap flua can use a
module when writing scripts. On my build machine, the consequence on
our binary size is an increase from around 1.6M -> 1.9M, which isn't
really that bad.

.lua modules can install into their usual path below $WORLDTMP/legacy
and we'll pick them up automagically by way of the ctor that sets up
LUA_PATH early on.

This re-lands bootstrap module support with a more sensible subset, and
after having verified that it cross-builds fine on macOS and Linux -- we
cannot do libfreebsd on !FreeBSD because it's more system header
dependant. We also need to bootstrap libmd to bring in libhash, and
libucl + libyaml.

Reviewed by: bapt, emaste (both previous version)
Differential Revision: https://reviews.freebsd.org/D51890

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