Excpose sink functions and structures
- src/mimemaker.c (write_buffer, sink_std_write), (write_buffer_for_cb, sink_encryption_write): Remove staticness.
- src/mimemaker.h (write_buffer, sink_std_write), (write_buffer_for_cb, sink_encryption_write): Add prototypes.
- src/mimemaker.c, src/mimemaker.h (sink_s, sink_t): Move declaration into header.
- Those will be used in the ribbon callbacks