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.

Tested by: Petr Salinger <Petr.Salinger@seznam.cz>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week