GH975 Add ex_data functions for X509_STORE

Add X509_STORE_{set,get}_ex_data() function and
X509_STORE_get_ex_new_index() macro.

X509_STORE has ex_data and the documentation also mentions them but they
are not actually implemented.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>