lib/posix-process/signal: Add signal descriptors and primitives

lib/posix-process/signal: Add signal descriptors and primitives

Introduce signal descriptors into `struct posix_process` and
`struct posix_thread`. Introduce primitives for various signal
operations such as initialization, allocation, and queueing.

Initialize signal descriptors upon creating a new process or
a thread.

Checkpatch-Ignore: LONG_LINE
Checkpatch-Ignore: MACRO_ARG_REUSE
Checkpatch-Ignore: SPACING
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