Make JSON audit logging a configurable option

Remove compile-time setting for generating audit logs
as JSON, creating a new config option (SecAuditLogFormat).
sec_audit_logger is now a wrapper for sec_audit_logger_json
or sec_audit_logger_native. This has the disadvantage of
making the audit log generation code harder to maintain,
but the logger function itself now is no longer pepper
with binary branches.

apache2/modsecurity.h

@@ -519,6 +519,9 @@ struct directory_config {
     /* AUDITLOG_SERIAL (single file) or AUDITLOG_CONCURRENT (multiple files) */
     int                  auditlog_type;
 
+    /* AUDITLOGFORMAT_NATIVE or AUDITLOGFORMAT_JSON */
+    int                  auditlog_format;
+
     /* Mode for audit log directories and files */
     apr_fileperms_t      auditlog_dirperms;
     apr_fileperms_t      auditlog_fileperms;