Make "tinc add" idempotent.

When calling "tinc add" multiple times with the same variable and value,
make sure only one unique line is added to the configuration file.

doc/tinc.8.in

@@ -88,6 +88,7 @@ To set a variable for a specific host, use the notation
 .Ar host Ns Li . Ns Ar variable .
 .It add Ar variable Ar value
 As above, but without removing any previously existing configuration variables.
+If the variable already exists with the given value, nothing happens.
 .It del Ar variable Op Ar value
 Remove configuration variables with the same name and
 .Ar value .

doc/tinc.texi

@@ -2293,6 +2293,7 @@ To set a variable for a specific host, use the notation @var{host}.@var{variable
 @cindex add
 @item add @var{variable} @var{value}
 As above, but without removing any previously existing configuration variables.
+If the variable already exists with the given value, nothing happens.
 
 @cindex del
 @item del @var{variable} [@var{value}]

src/tincctl.c

@@ -1610,6 +1610,11 @@ static int cmd_config(int argc, char *argv[]) {
 				}
 				set = true;
 				continue;
+			// Add
+			} else if(action > 0) {
+				// Check if we've already seen this variable with the same value
+				if(!strcasecmp(bvalue, value))
+					found = true;
 			}
 		}
 
@@ -1642,7 +1647,7 @@ static int cmd_config(int argc, char *argv[]) {
 	}
 
 	// Add new variable if necessary.
-	if(action > 0 || (action == 0 && !set)) {
+	if((action > 0 && !found)|| (action == 0 && !set)) {
 		if(fprintf(tf, "%s = %s\n", variable, value) < 0) {
 			fprintf(stderr, "Error writing to temporary file %s: %s\n", tmpfile, strerror(errno));
 			return 1;