tfhash: New template function to calculate the SHA1 of its input

This patch adds a new template function called $(hash) which can be used
to calculate the SHA1 hash of its inputs. As it uses the SHA1() function from
OpenSSL, it is only available if syslog-ng is compiled using --enable-ssl.
    
One use case for it can be when you'd like to anonymize sensitive parts of the
log message (eg. the user name) you parsed out using patterndb before storing
or forwarding it for further analysis but would like to retain the ability to
correlate messages along this value. Beware though that such simple anonymization
can be broken if there's enough data available, see
http://arxiv.org/pdf/cs/0610105v2.pdf for references and examples.
    
Thanks for the original idea to Martin Grauel.
  
Signed-off-by: Peter Gyongyosi <gyp@balabit.hu>
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[see the patch itself at https://github.com/gyp/syslog-ng-3.4/commit/5c6e97d94694eef2aa76fa61a1c8928269f781ff ]