Use new argument inset code for environments

This removes another instance of the limitation solved by file format 446:
The order of optional and required arguments is not fixed anymore.
Write fixed encoding to LyX file (bug #9178)

If the user gave the -fixednec argument to tex2lyx then set that encoding as
LaTeX input encoding in the generated LyX file. Otherwise, included .bib files
could be interpreted using a wrong encoding.
Handle unknown table column specifiers (bug #9311)

Previously tex2lyx did ignore table columns if the column specifier was
unknown. This can lead to data loss, and now it does not ignore these columns,
but assumes that the column specifiers use only once character. This can still
lead to data loss, but less often.