aim of this patch is to be able to implement sigevent parameter of POSIX 
lio_listio(). It is applied on my previous patch implementing signal on an AIO 
request completion.

I create a new request type IO_CMD_EVENT to put in an io_submit() iocbs, 
parameters are signo and pid. All following requests of the list submited by 
the io_submit() are  in the same group, and when all these requests are 
completed a signal signo is sent to pid (if pid = 0, it means itself, otherwise 
it's a thread id). We can have several groups of requests in one io_submit(), 
each separated by a IO_CMD_EVENT. A group can be broken without starting a new 
one with IO_CMD_EVENT with a signo equal to 0.

Example:

iocbs[...] is :

R0: IO_CMD_EVENT signo = SIGRTMIN + 1, pid = 0
R1: IO_CMD_PREAD ...
R2: IO_CMD_PREAD ...
R3: IO_CMD_EVENT signo = SIGRTMIN + 2, pid = 1238
R4: IO_CMD_PWRITE ...
R5: IO_CMD_PWRITE ...
R6: IO_CMD_FSYNC ...
R7: IO_CMD_EVENT signo = 0, pid = 0
R8: IO_CMD_PREAD ...
R9: IO_CMD_PREAD ...
...
io_submit(..., iocbs)

when R1 and R2 are completed, SIGRTMIN+1 is sent to the process itself
when R4 and R5 are completed, SIGRTMIN+2 is sent to the thread 1238
R6, R8, R9, ... don't generate signal.