Add CRYPTO_EX_DATA; remove EC_EXTRA_DATA

Add CRYPTO_EX_DATA add EndC_KEY_[gs]et_method, From Roumen Petrov.
Had to add various exdata calls to init/copy/free the exdata.
Had to remove const from some EC functions because exdata isn't
const-correct. :(
Also remove EC_EXTRA_DATA and use a union to hold the possible
pre-computed values and an enum to tell which value is in the
union. (Rich Salz)

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>

doc/crypto/CRYPTO_get_ex_new_index.pod

@@ -45,8 +45,7 @@ The specific structures are:
     X509_STORE_CTX
     DH
     DSA
-    ECDH
-    ECDSA
+    EC_KEY
     RSA
     ENGINE
     UI