Hide public symbols in evp.h

largely mechanically done by the guentherizer 9000

ok tb@

Check the return value of EVP_CIPHER_CTX_reset()

The function call can't actually fail, but all other calls check its
return value.

ok joshua jsing

Clean up use of EVP_CIPHER_CTX_{legacy_clear,cleanup} in EVP_OpenInit

ok tb@

Use EVP_CIPHER_CTX_legacy_clear() internally

ok jsing

Merge p_open and p_seal into p_legacy

discussed with jsing

Fold p_dec.c and p_enc.c into a new p_legacy.c

discussed with jsing