lib/posix-process: Add uk_posix_process_create_pthread()

lib/posix-process: Add uk_posix_process_create_pthread()

Provide a function to attach a thread to the current process. This is
intended to be exclusively used by app-elfloader when configured with
initrd, in order to assign the container thread it creates to the init
process.

Move uk_process_kill() to the internal API, and rename to avoid using
the naming convention of public functions, and deprecate the unused
uk_posix_process_create().

Checkpatch-Ignore: REPEATED_WORD
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