MFC r253527:
Move the convert_sigevent32() utility function into freebsd32_misc.c
for consumption outside the vfs_aio.c.

For SIGEV_THREAD_ID and SIGEV_SIGNAL notification delivery methods,
also copy in the sigev_value, since librt event pumping loop compares
note generation number with the value passed through sigev_value.
MFC r253528:
The freebsd32_lio_listio() compat syscall takes the struct sigevent32.
Regenerate.
MFC r253529:
Wrap kmq_notify(2) for compat32 to properly consume struct sigevent32
argument.