Improve the ibuf API by adding these functions:

Improve the ibuf API by adding these functions:
Functions extending ibuf_add to work with more specific data types
  ibuf_add_buf, ibuf_add_zero, ibuf_add_n8, ibuf_add_n16, ibuf_add_n32,
  ibuf_add_n64
Functions replacing ibuf_seek where data at a specific offset is modified
  ibuf_set, ibuf_set_n8, ibuf_set_n16, ibuf_set_n32, ibuf_set_n64
Functions to check, get and set the filedescriptor stored on the ibuf
  ibuf_fd_avail, ibuf_fd_get, ibuf_fd_set
and ibuf_data() to access the data buffer, to be used together with ibuf_size()

On top of this add an optimized imsg_compose_ibuf() where an ibuf is wrapped
into an imsg in an efficent way.

Finally remove msgbuf_drain since it is not used by anything outside of
the ibuf code. Because of this removal bump the major of libutil.

Remove ibuf_data() in iked since the same function is now provided by libutil.
OK tb@