Change defines into inline functions

To prevent bugs caused by pointers having the wrong type (e.g. uint32_t
instead of uint8_t), some macros are changed into inline functions so that
the normal type-checking is performed.

The macros MYSQL_GET_ERRCODE, MYSQL_GET_STMTOK_NPARAM, MYSQL_GET_STMTOK_NATTR,
and MYSQL_GET_NATTR were not changed, because they may be too specific to
be present in a general purpose header in the first place.

query_classifier/qc_mysqlembedded/qc_mysqlembedded.cc

@@ -58,6 +58,11 @@
 #include <maxscale/debug.h>
 #include <maxscale/log_manager.h>
 #include <maxscale/query_classifier.h>
+// <maxscale/protocol/mysql.h> assumes it is being compiled agains Connector-C,
+// so we need to make certain Connector-C constants visible.
+#define MYSQL_COM_QUIT        COM_QUIT
+#define MYSQL_COM_INIT_DB     COM_INIT_DB
+#define MYSQL_COM_CHANGE_USER COM_CHANGE_USER
 #include <maxscale/protocol/mysql.h>
 #include <maxscale/gwdirs.h>