Move quote triming function to common place

Function for removing MySQL quote characters surrounding a
string moved from Cache's rules.cc to common place and renamed.

The function was now moved to mysql_utils.[h|c], which is in
maxscale-common, and not to the perhaps more appropriate place
protocol/mysql.h, as the latter would have implied that MySQLCommon
would have to be linked to, which actually might be the right thing
to do. However, that has to wait until protocol/mysql.h gets an
overhaul.

include/maxscale/mysql_utils.h

@@ -31,4 +31,17 @@ char* mxs_lestr_consume(uint8_t** c, size_t *size);
 
 MYSQL *mxs_mysql_real_connect(MYSQL *mysql, SERVER *server, const char *user, const char *passwd);
 
+/**
+ * Trim MySQL quote characters surrounding a string.
+ *
+ *   'abcd' => abcd
+ *   "abcd" => abcd
+ *   `abcd` => abcd
+ *
+ * @param s  The string to be trimmed.
+ *
+ * @note The string is modified in place.
+ */
+bool mxs_mysql_trim_quotes(char *s);
+
 MXS_END_DECLS