mx_mysql: Fix warning in mx_mysql_option_set_default_file

If the file is not readable, the warning is now saved to be retrievable
by the caller instead of being emitted. The warning text has been
shortened from:

    MySQL ignoring defaults file: euidaccess("/etc/mxq/mysql_ro.cnfX", R_OK) failed: No such file or directory
    MySQL falling back to mysql default config search path.

to

    /etc/mxq/mysql_ro.cnfX: No such file or directory - falling back to mysql default config search path

mx_mysql.c

@@ -41,6 +41,19 @@ static void mx_mysql_save_error(const char *err) {
     strncpy(mx_mysql_last_error, err, len+1);
 }
 
+__attribute__ ((format (printf, 1, 2)))
+static void mx_mysql_save_error_va(const char *restrict fmt, ...) {
+    char *s;
+    va_list ap;
+    va_start(ap, fmt);
+    int res = vasprintf(&s, fmt, ap);
+    va_end(ap);
+    if (res == -1)
+        return;
+    mx_mysql_save_error(s);
+    free(s);
+}
+
 char *mx_mysql_error(void) {
     return mx_mysql_last_error == NULL ? "no error information" : mx_mysql_last_error;
 }
@@ -614,8 +627,7 @@ int mx_mysql_option_set_default_file(struct mx_mysql *mysql, char *fname)
     mx_assert_return_minus_errno(mysql, EINVAL);
 
     if (fname && (*fname == '/') && (euidaccess(fname, R_OK) != 0)) {
-        mx_log_warning("MySQL ignoring defaults file: euidaccess(\"%s\", R_OK) failed: %m", fname);
-        mx_log_warning("MySQL falling back to mysql default config search path.");
+        mx_mysql_save_error_va("%s: %m - falling back to mysql default config search path", fname);
         return -errno;
     }