Major reworking of org.conman.fsys

I decided to get rid of the FD type and use LUA_FILEHANDLEs exclusively
(which are really FILE **).  I never really used the fsys.open(),
fsys.read(), fsys.write() and fsys.close() calls (and other metatable
calls).  This simplifies things here.
Rename org.conman.net.socketfd() to org.conman.net.socketfile()

Because org.conman.fsys no longer returns a file descriptor, we no longer
have file descriptors, but we still have LUA_FILEHANDLES.  So fix
org.coman.fsys.socketfd() to accept LUA_FILEHANDLES and while we're at it,
rename it.
Bug fix---didn't properly set the environment for fsys.pipe() and fsys.dup()

Or any function that creates a LUA_FILEHANDLE.  The fix---copy the
environment from io.open() to all the functions here.
Rework fsys.pipe() and fsys.openfd() to be safer.

To be sure memory is reclaimed properly, create our return data first, with
NULL values.  Then do our operations.
Clarify some code.
Set the buffer mode on a dup'ed file.

I can't really find anything about resetting the buffer of a FILE *. The
intent here is to reset the buffer so that any pre-existing data doesn't
affect the copy (since we're closing and opening the underlying file).  I'm
not concerned with losing any memory here since Lua never specifies an
actual buffer (it always calls setvbuf() with a NULL buffer pointer) and I
expect the underlying C library to Do The Right Thing (TM Spike Lee).

Under Unix, if the file descriptor refers to a TTY, the buffering is lined
based; else it's fully buffered.
Update the README.
Bug fix---dup2() on the same file doesn't exactly work, now does it?
I don't think I need the buffering.
Rework fsys.dup().

On second thought, I might need to use actual file descriptors (I'm thinking
of redirecting with network sockets).  So, let's use either FILE* or integers
here.
Reintoduce fsys._STDIN, fsys._STDOUT and fsys._STDERR.

On second thought, since I can now use file descriptors for fsys.dup(), I
figured I would go back and add these.
Reorder things so I don't pollute the public C namespace.