set parallel_build_level to 1 in doc mode

The current autobuild support for documentation generation is not
compatible with parallel builds, so disable it for now.
re-allow parallel build for documentation, autobuild has been fixed
release 1.8.2.b1
modify the gitorious handler to remove http fallback but set a retry count
use exclusively Autobuild message-display API
make Autoproj.silent? and .silent= simply forward to Autobuild.silent*
remove libxml and libxslt from default.osdeps, we don't need nokogiri anymore
install rubygems-integration by default on debian

This allows to install Debian packages instead of gems, but still
have the gems dependencies fullfilled.
define env_source_(after|before) instead of env_source_file

env_source_file is kept for backward compatibility to call
env_source_after
add the current gem_home to GEM_PATH

This is the first step towards reusing autoproj installations
use the importer's repository_id call to generate the remote directory name

This fixes unstable names (the name changes over different runs)
fix handling of osdep overrides in the non-force case

In this case, the osdep name was always used as the source package name,
instead of honoring the setting given to add_osdep_override
make AUTOPROJ_OS always override the operating system

It now bypasses caching, which was making AUTOPROJ_OS inoperative
in some operations
osdep: fix having non-specific OS entries considered even if a specific OS entry exists

The issue is with an entry like

  ubuntu: package
  default: gem

Currently, both the package and the gem would have been considered, while
(obviously) only the package should have been
load osdeps early, so that the autobuild files can know which packages are available

This is needed so that one can automatically exclude a package if an
osdep is not present.
auto-exclude packages for which an osdep is not available on the current OS
osdep: priority of resolution of OS-specific entries was depending on the entry order, fix
test: remove unneeded mocking of #names on package managers
add the experimental autoproj-inspect tool
test: fix test w.r.t. raising MissingOSDep instead of ConfigError