/*
 * call-seq:
 *    Filesystem.mount_point(file)
 *
 * Returns the mount point of the given +file+, or itself if it cannot be
 * determined.
 *
 * Example:
 *
 *    Filesystem.mount_point('/home/djberge/some_file.txt') => '/home'
 */
static VALUE fs_mount_point(VALUE klass, VALUE v_file){