printsiginfo: add SIGSYS decoding

* configure.ac (AC_CHECK_MEMBERS): Check for siginfo_t.si_syscall.
* signal.c (SYS_SECCOMP): Define if not yet defined.
(sigsys_codes): new xlat structure.
(printsiginfo): Decode SIGSYS.
diff --git a/configure.ac b/configure.ac
index 682f54e..39832ef 100644
--- a/configure.ac
+++ b/configure.ac
@@ -256,7 +256,8 @@
 AC_CHECK_MEMBERS([struct utsname.domainname],,, [#include <sys/utsname.h>])
 
 AC_CHECK_MEMBERS([struct sigevent._sigev_un._pad,
-		  struct sigevent.__pad],,, [#include <signal.h>])
+		  struct sigevent.__pad,
+		  siginfo_t.si_syscall],,, [#include <signal.h>])
 
 AC_CHECK_TYPES([struct flock64],,, [#include <fcntl.h>])