lib/posix-process: Create PID1 from the thread passed in init

lib/posix-process: Create PID1 from the thread passed in init

Update the instantiation of INIT_PID to use the thread passed in the
init context. This allows excluding unikraft's init thread from the
process when libukboot creates a separate thread for main().

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
GitHub-Closes: #1620