lib/posix-process/signal: Add kill() and sigqueueinfo()

lib/posix-process/signal: Add kill() and sigqueueinfo()

Add syscalls for signalling. Internally these share common
abstractions.

The kill syscalls are used for sending signals to processes,
process groups, or threads:

* kill() allows sending a signal to a process group or process.
* tkill() allows sending a signal to a thread.
* tgkill() allows sending a signal to a thread in thread
  group.

The siqeueueinfo syscalls are similar to kill but additionally
allow to accompany the signal with data:

* rt_sigqueueinfo() allows sending signal and data to a process.
* rt_tgsigqueueinfo() allows sending signal and data to a thread
  in thread group.

For more info see kill(2), tkill(2), rt_sigqueueinfo(2)

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Ioan-Teodor Teugea <ioan_teodor.teugea@stud.acs.upb.ro>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1248