allow drush files to be installed in standard locations

For the Aegir project, I'm considering creating Debian packages for both Provision and Drush so that we can easily create new platforms with automated scripts. To do this right, Provision would logically need to be installed in /usr/share/drush/provision (since it's a non-binary shared library):

       /usr/share
              This directory contains subdirectories with specific application
              data, that can be shared among different  architectures  of  the
              same  OS.   Often  one  finds  stuff  here  that used to live in
              /usr/doc or /usr/lib or /usr/man.

The system-wide configuration file should be in /etc/drush/drushrc.php:

       /etc   Contains  configuration  files  which  are local to the machine.
              Some larger software packages, like X11, can have their own sub‐
              directories  below  /etc.   Site-wide configuration files may be
              placed here  or  in  /usr/etc.   Nevertheless,  programs  should
              always  look  for these files in /etc and you may have links for
              these files to /usr/etc.

Indeed, there's already a TODO for that in the code:

  // TODO: This really should be a proper, system-wide location.
  $configs['drush'] = dirname(__FILE__) . '/../drushrc.php';

So I attach a patch to fix both those issues. It doesn't enforce anything: you can still use ~/.drush for rc files or modules, but it adds support for those directories. It does, however, drop support for drushrc in the drush directory. This is trivial to add back, I think.

This would greatly facilitate packaging and code separation. It will also facilitate interoperability with other systems, as the Drush files will be in a consistent, predictable directory (/etc or /usr/share).

This is a must for Provision, especially for /etc/drush because we will soon need to have "server verification" in which some system-wide settings need to be written to a configuration file. That would be /etc/drush/drushrc.php.

For third party packages, it is also essential that a system-wide location is available to load commands from. ~/.drush will not work because the commands will be available to only one user (the same could apply to drushrc.php).

I am therefore marking this as critical so it makes it into 2.0, which provision 0.2 and later will depend upon.