List of things that would be nice to change if we ever get a chance to bump libc ABI

* Size of struct sigevent (see #666454).

* id_t: maybe switch to uint64_t as in kFreeBSD, see

* clock_t: maybe switch to unsigned as in kFreeBSD.

* time_t: maybe switch to int64_t on amd64, as in kFreeBSD (current type is "long" which matches size and signedness but requires special-casing in bits/typesizes.h).

* Shorten "sun_path" in "struct sockaddr_un" from 108 to 104 to match kernel definition (and remove associated kludges), see #645527.