lib/posix-process: Consolidate assignment of pthread parent and self

lib/posix-process: Consolidate assignment of pthread parent and self

uk_posix_process_create() is missing the assignment of pthread_self.
Instead of having wrappers perform these assignments of parent and
self, move these assignments to the core functions that create and
release threads.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1627