Texi2html: Set the TOC depth using a command-handler

Texi2html: Set the TOC depth using a command-handler

It seems that command_handler_process is the correct command handler
to set the TOC depth (after parsing all input files, but before any
possible output or TOC is created), depending on the texinfo variable
short_toc. This fixes my previous commit, where I had some workarounds,
because I didn't think of command_handler_process.