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/msc_logging.h

@@ -22,6 +22,9 @@
 #define AUDITLOG_SERIAL                      0
 #define AUDITLOG_CONCURRENT                  1
 
+#define AUDITLOGFORMAT_JSON                  0
+#define AUDITLOGFORMAT_NATIVE                1
+
 #define AUDITLOG_PART_FIRST                 'A'
 #define AUDITLOG_PART_HEADER                'A'
 #define AUDITLOG_PART_REQUEST_HEADERS       'B'