Make the fileds of mbedtls_net_context public

[gabor-mezei-arm]

As part of #4683, as a result of reviewing the programs there are uses of MBEDTLS_PRIVATE fields of the mbedtls_net_context structure. To avoid the access of the private fields make the fields of mbedtls_net_context public.

Accessed fields:

• fd

Affected files:

• ssl/mini_client.c

[gilles-peskine-arm]

#4842 made this field public (with a documented stipulation that this is only guaranteed on Unix/POSIX platforms).