conf: cast to unsigned char for character handling function

Character handling functions, like isspace(3), expect a value
representable as unsigned char or equal to EOF.  Otherwise the behavior
is undefined.

See https://wiki.sei.cmu.edu/confluence/display/c/STR37-C.+Arguments+to+character-handling+functions+must+be+representable+as+an+unsigned+char

conf/pam_conv1/pam_conv_y.y

@@ -72,7 +72,7 @@ line
 
     /* make sure we have lower case */
     for (i=0; $1[i]; ++i) {
-	$1[i] = tolower($1[i]);
+	$1[i] = tolower((unsigned char)$1[i]);
     }
 
     /* $1 = service-name */