audit plugin - record full query / document line length / make buffer configurable

Hey,

the audit plugin "should" record every part of the query. the current implementation is using a static buffer to shorten the query part. since an audit is used to detect problems or hacking attempts, the audit plugin is unusable. the current size of the buffer is 768 and should be dynamic by default or atleast configurable.
especially on hacking attacks the last part (where clause) is modified but this part is not part of the audit line. in addition this should be added to the documentation.

https://github.com/MariaDB/server/blob/10.1/plugin/server_audit/server_audit.c#L1018