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

libc/include/signal.h

@@ -66,6 +66,8 @@ extern const char* const sys_signame[]; /* BSD compatibility. */
 typedef __sighandler_t sig_t; /* BSD compatibility. */
 typedef __sighandler_t sighandler_t; /* glibc compatibility. */
 
+#define si_timerid si_tid /* glibc compatibility. */
+
 #if defined(__LP64__)
 
 struct sigaction {