Move gengetopt input files to m4 directory. This separates the gengetopt input directory from its output directory. All gengetopt m4 file are moved to the new m4/gengetopt directory. As a consequence of this cleanup, we may get rid of the .gitignore file for the generated ggo files, which is a good thing as it was hard to keep it up to date.
recv: Use buffer tree API unconditionally. This allows to kill the -B option.
Add option for activating new buffer tree code to para_recv and para_write. ATM this option is ignored.
Use symbolic names for loglevels and clean up the ggo mess. We now use m4 to generate the ggo files, which allows to get rid of a lot of duplicated command line options and improves the readability of the man pages.