Check Access Permissions

/*
 * check_access - check if the user has read permission to a
 *                given file. If not, check where the problem lies.
 *
 * Background: In order to read from a file, we must have
 *             execute permission in all directories leading
 *             to this file, as well as read access to the
 *             file itself. For example, to read from file
 *             "/etc/config/blabla.conf", we must have X
 *             permission on '/etc' and on '/etc/config',
 *             as well as R permission on
 *             "/etc/config/blabla.conf" itself.
 *
 * Algorithm: split the file path to its directories, make
 *            sure the directory exists and that the user
 *            has execute permission to all directories.
 *            Finally check that the file exists, and that
 *            the user has read permission to the file itself.
 */