Change BSTR() from a macro producing a compound literal to an inline
function returning the same value. This works for all existing uses,
and avoids a warning from BSTR(NULL) (the macro expansion contained
strlen(NULL); this was valid code because the strlen call was never
evaluated, but still triggered a GCC warning).
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.
