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: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>