Initialize main thread pthread structure

Initialize main thread pthread structure

It seems some steps were missing in pthread structure initialization.
This patch does the following:
- zeroes pthread memory (previously potentially contained garbage after
uk_mem_align allocation)
- initializes tsd
- sets detach_state to joinable

Signed-off-by: Aleksandr Iashchenko <Aleksandr.Iashchenko@opensynergy.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Robert Kuban <robert.kuban@opensynergy.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #59