lib/posix-process: Add process table

lib/posix-process: Add process table

Add process table that maps `pid_t` to `struct posix_process`.
Add process lookup function and process & thread iterators.

Checkpatch-Ignore: COMPLEX_MACRO
Checkpatch-Ignore: MACRO_ARG_REUSE
Checkpatch-Ignore: SUSPECT_CODE_INDENT
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