1) Two new arguments (char *val, unsinged int maxlen) added to the

1) Two new arguments (char *val, unsinged int maxlen) added to the
following functions and macros:
exif_entry_get_value
exif_entry_get_value_brief
exif_mnote_data_get_value
exif_mnote_data_pentax_get_value
macros:
exif_content_get_value
exif_content_get_value_brief

The functions return either NULL (in case of error) or val.

2) All static variables eliminated.

3) memset(val, 0, maxlen); is in all functions, followed by maxlen--;
 which ensures all returned strings will always be NULL-terminated.

All this should make the library thread-safe.