linux(4): Refactor signal send methods.

Created a couple of helpers to send signals to the specific thread or to
the whole process. Use helpers in the corresponding syscalls.
This fixes the confusion where a signal destined for a whole process
was sent to a specific thread and vice versa.
There is an exclusion for the linux_kill() syscall that takes a pid
argument and should send a signal to the whole process, but I know
at least one example where kill() takes tid.

MFC after: 2 weeks