lib/posix-process: Add helpers for current process and thread

lib/posix-process: Add helpers for current process and thread

Add uk_pprocess_current() uk_pthread_current() helpers. For now
these are part of the private API.

Checkpatch-Ignore: GLOBAL_INITIALISERS
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