Commit df899f59be removing a write outside a buffer triggered another
problem, as for some reason the code did not 0-terminate its read
buffer in the specific case that it had encountered an EOF, and as a
result could parse contents left in the buffer for a second time.
Usually this resulted in parsing error messages. Fix the problem by
rewriting the offending code in a less hacky form.
Analogously to the previous commit, move path handling logic for
loading external vobsub files from mplayer.c to find_subfiles.c.
Based on a commit from Clément Bœsch but fixed and simplified.
Move "struct bstr" definition from ebml.h to its own header and add
some utility functions/macros. Change length field type from int to
size_t and adjust using code accordingly.
Partially based on a patch from Anton Khirnov.
