enum imsg_type and not an int

-don't buf_free() on imsg_add() errors, it already does that for us
-use imsg_close() instead of buf_close(), does error handling already

does the error checking for us.

-kill the _pid flavors of imsg_create and imsg_compose, and just add pid as
argument to those
-use imsg_create in imsg_compose instead of duplicating code
-check for datalen overflow

it is just capable of answering (s)ntp4 requests with the local time
for now.
imsg/buffer and logging framework from bgpd, ntp protocol hackery
with Alexander Guy