Add si_timerid to <signal.h>.

The kernel uses the very misleading name "si_tid", but glibc uses the more
intention-revealing "si_timerid". We should let people use that.

(Added because I wanted to improve SI_TIMER siginfo_t dumping in strace.)

Change-Id: Ib626cdd3b57a6afb276a15753a237b4e81ec45e3