lib/ukboot: Pass tmain to init context

lib/ukboot: Pass tmain to init context

Populate the init context with the thread that executes main().
When LIBUKBOOT_MAINTHREAD is selected, this allows libposix-process
to create INIT_PID from the main thread while excluding Unikraft's
init thread from the process.

Move the initialization of the main() thread before executing inittab.
With that change, update the parameters main_thread() function to pass
a pointer to ictx, so that it's possible for args to be updated by
inittab.

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