signals: add rt_tgsigqueueinfo syscall

sys_kill has a counterpart sys_tgkill which allows to send signals to
a particular thread. sys_rt_sigqueueinfo is lacking such a counterpart.

Aside of the asymetry it is a show stopper for migrating applications
from other unix-alike RTOSes.

The following patch series implements rt_tgsigqueueinfo and hooks it
up for x86.

Find below the raw documentation.

Thanks,

	tglx
----

NAME
       rt_tgsigqueueinfo - Send signal information to a signal to a thread

SYNOPSIS
       long sys_rt_tgsigqueueinfo (int tgid, int tid, int sig, siginfo_t *uinfo);

DESCRIPTION

       rt_tgsigqueueinfo sends signal sig information uinfo to the
       thread with the thread ID tid in the thread group tgid.  (By
       contrast, rt_sigqueueinfo(2) can only be used to send a signal
       info to a process (i.e., thread group) as a whole, and the
       signal will be delivered to an arbitrary thread within that
       process.)

RETURN VALUE

       rt_tgsigqueueinfo returns 0 on success; otherwise,
       rt_sigqueueinfo returns one of the errors listed in the
       "Errors" section.

ERRORS
       -EFAULT
              An invalid value for uinfo was specified.

       -EINVAL
	      An invalid TID, TGID or signal was specified.

       -EPERM 
              Permission denied.  For the required permissions,
              see rt_sigqueueinfo(2).

       -ESRCH 
       	      No process with the specified thread ID and thread group
              ID exists.