Clarify the operation of the SessionCryptoPassphrase directive as raised by rpluem...

Clarify the operation of the SessionCryptoPassphrase directive as raised by rpluem. When
SessionCryptoCertificateFile is set, asymmetrical encryption will be used, and SessionCryptoPassphrase
will be interpreted as the passphrase protecting the private key. When SessionCryptoCertificateFile
is not set, symmetrical encryption is used, and SessionCryptoPassphrase will contain the key to
use. Make sure that the engine parameter is properly passed into the crypto functions, and fix a
missing cleanup on an error case.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@645186 13f79535-47bb-0310-9956-ffa450edef68