Set tid to 0 for the main thread

Set tid to 0 for the main thread

This commit ads in the Config.uk the following default selection:
`LIBPOSIX_PROCESS_INIT_PIDS`, which allows pid,tid assignation
However, there is a bug in the assignment of tids that
makes the tid of the main thread not available when the
LIBC TCB (i.e. pthread structure) is initialized. The
currently adopted solution is to hardcode the tid to 0.

Signed-off-by: Dragos Iulian Argint <dragosargint21@gmail.com>
Signed-off-by: Robert Kuban <robert.kuban@opensynergy.com>
Reviewed-by: Florin Postolache <florin.postolache80@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@unikraft.io>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Robert Kuban <robert.kuban@opensynergy.com>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #9