config-parser: Honor XDG_CONFIG_DIRS.

config-parser: Honor XDG_CONFIG_DIRS.

This set of changes adds support for searching for a given config file
in the directories listed in $XDG_CONFIG_DIRS if it wasn't found in
$XDG_CONFIG_HOME or ~/.config.  This allows packages to install custom
config files in /etc/xdg/weston, for example, thus allowing them to
avoid dealing with home directories.

To avoid a TOCTOU race the config file is actually open()ed during the
search.  Its file descriptor is returned and stored in the compositor
for later use when performing subsequent config file parses.

Change-Id: If473edf71839434eb0de88061a3ceca593a934ad
Signed-off-by: Ossama Othman <ossama.othman@intel.com>