Add strlist_contains()

Add a function to know if a string list, using some separator, contains
a specific string.

app/tests/test_strutil.c

@@ -287,6 +287,18 @@ static void test_parse_integer_with_suffix(void) {
     assert(!ok);
 }
 
+static void test_strlist_contains(void) {
+    assert(strlist_contains("a,bc,def", ',', "bc"));
+    assert(!strlist_contains("a,bc,def", ',', "b"));
+    assert(strlist_contains("", ',', ""));
+    assert(strlist_contains("abc,", ',', ""));
+    assert(strlist_contains(",abc", ',', ""));
+    assert(strlist_contains("abc,,def", ',', ""));
+    assert(!strlist_contains("abc", ',', ""));
+    assert(strlist_contains(",,|x", '|', ",,"));
+    assert(strlist_contains("xyz", '\0', "xyz"));
+}
+
 int main(int argc, char *argv[]) {
     (void) argc;
     (void) argv;
@@ -304,5 +316,6 @@ int main(int argc, char *argv[]) {
     test_parse_integer();
     test_parse_integers();
     test_parse_integer_with_suffix();
+    test_strlist_contains();
     return 0;
 }