PHO_Display: Improve verbose level mgt

New constants give min/max level.
Add dec_verbose() method
inc_verbose()/dec_verbose() are now limited to max/min levels.
PHO_Getopt: Fix error messages
Improve build system

Improve Makefile
Translate comments
Improve xref build
Improve doc generation
Fix phool.psf to comply with new syntax
PHO_File: Incorporate changes from Automap

Modified combine_path()
New methods : trailing_separ, is_absolute_path, mk_absolute_path.
Fix PHO_HTTP

PHO_Util::is_web() is renamed to env_is_web()