[MINOR] buffers: add buffer_cut_tail() to cut only unsent data

[MINOR] buffers: add buffer_cut_tail() to cut only unsent data

This function is used to cut the "tail" of a buffer, which means strip it
to the length of unsent data only, and kill any remaining unsent data. Any
scheduled forwarding is stopped. This is mainly to be used to send error
messages after existing data. It does the same as buffer_erase() for buffers
without pending outgoing data.