Merge various improvements from the OpenSSL 1.1 branch,
which is still under a free license.
* document EVP_MD_CTX_set_flags, EVP_MD_CTX_clear_flags, EVP_MD_CTX_test_flags
* document EVP_MD_flags, EVP_MD_CTX_md_data
* document EVP_MD_CTX_pkey_ctx, EVP_MD_CTX_set_pkey_ctx
* correct arg type of EVP_MD_CTX_size, EVP_MD_CTX_block_size, EVP_MD_CTX_type
* more information about EVP_MD_CTX_ctrl
* add missing <string.h> and correct one variable type below EXAMPLES
* two orthographic improvements
with a few wording tweaks by me