lib/posix-process: Correct MAX_PID config option

lib/posix-process: Correct MAX_PID config option

This commit changes the tid_map size to reflect the kernel
reservation of the 0 TID/PID value. Before, the first thread
would set the 0 position in all used structures. Now, because
it will be already set, we need to make room for another.

Signed-off-by: Florin Postolache <florin.postolache.of@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Andra Paraschiv <andra@unikraft.org>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #885