Add isc_buffer_setmctx() and isc_buffer_clearmctx() function

Add isc_buffer_setmctx() and isc_buffer_clearmctx() function

Add two extra functions needed by StreamDNS:

1. isc_buffer_setmctx() sets the buffer internal memory context, so we
   can use isc_buffer_reserve() on the buffer.  For this, we also need
   to track whether the .base was dynamically allocated or not.  This
   needs to be called after isc_buffer_init() and before first
   isc_buffer_reserve() call.

2. isc_buffer_clearmctx() clears the buffer internal memory context, and
   frees any dynamically allocated buffer.  This needs to be called
   after the last isc_buffer_reserve() call and before calling the
   isc_buffer_invalidate()