+ read_header_auto process them automatically,
+ read_header_x_raw when a special header is read, return
+ HEADER_SUCCESS_EXTENDED without actually
+ processing the header,
+ read_header_x_global when a POSIX global header is read,
+ decode it and return HEADER_SUCCESS_EXTENDED.
+
+ You must always set_next_block_after(*return_block) to skip past