The following flags can be passed in the flags parameter.
If PKCS7_DETACHED is set then cleartext signing will be used,
this option only makes sense for signedData where PKCS7_DETACHED
is also set when PKCS7_sign() is also called.
If the PKCS7_TEXT flag is set MIME headers for type text/plain
are added to the content, this only makes sense if PKCS7_DETACHED
is also set.
If the PKCS7_STREAM flag is set streaming is performed. This flag should
only be set if PKCS7_STREAM was also set in the previous call to
PKCS7_sign() or PKCS7_encrypt().
If cleartext signing is being used and PKCS7_STREAM not set then
the data must be read twice: once to compute the signature in PKCS7_sign()
and once to output the S/MIME message.
If streaming is performed the content is output in BER format using indefinite
length constructuted encoding except in the case of signed data with detached
content where the content is absent and DER format is used.