Fix struct sigevent decoding for musl

Fix struct sigevent decoding for musl

Do not rely on "struct sigevent.__pad" being located at the same address
as "struct sigevent.sigev_notify_thread_id", it's not the case with musl
libc.  Do not rely on struct sigevent definition at all to access
sigev_notify_thread_id.

* configure.ac (AC_CHECK_MEMBERS): Remove struct sigevent._sigev_un._pad
and struct sigevent.__pad.
* sigevent.h: New file.
* Makefile.am (strace_SOURCES): Add it.
* print_sigevent.c: Include it.
* print_sigevent.c (struct_sigevent): Remove.
(print_sigevent): Update all struct_sigevent users.
* tests/timer_create.c: Include "sigevent.h".
(main): Use struct_sigevent instead of struct sigevent,
all struct sigevent clients changed.