The posix module is subdivided according to C headers; for instance,
posix.unistd contains routines available from unistd.h, such as
chown(2).

A quirk of our implementation is that each of the modules is a direct
entry in the global table. That is, there is no "posix" table.
Instead, "posix.foo" and "posix.bar.baz" are both top-level tables.
This is surprising and goes against Lua's shorthand of using "." to
access keys in a table. lua-posix also doesn't work this way.

Rework things so that "posix" and "posix.sys" are proper tables.
Existing flua code which uses require() to bind posix submodules to a
name will be unaffected. Code which accesses them directly using
something like _G["posix.sys.utsname"].uname() will be broken, but I
don't think anything like that exists. In particular, it is now
possible to call posix.sys.utsname.uname() without any require
statements.