string-util: add a mechanism for strextend_with_separator() for specifying "ignore" arguments

in strv_new() we have STRV_IGNORE for skipping over an argument in the
argument list. Let's add the same to strextend_with_separator():

strextend_with_separator(&x, "foo", POINTER_MAX, "bar");

will result in "foobar" being appended to "x". (POINTER_MAX Which is
different from NULL, which terminates the argument list).

This is useful for ternary op situations.

(We should probably get rid of STRV_IGNORE and just use POINTER_MAX
everywhere directly, but that's for another time.)

src/test/test-string-util.c

@@ -262,6 +262,12 @@ TEST(strextend) {
         ASSERT_STREQ(str, "0123");
         assert_se(strextend(&str, "456", "78", "9"));
         ASSERT_STREQ(str, "0123456789");
+
+        assert_se(strextend(&str, "more", NULL, "huch"));
+        ASSERT_STREQ(str, "0123456789more");
+
+        assert_se(strextend(&str, "MORE", POINTER_MAX, "HUCH"));
+        ASSERT_STREQ(str, "0123456789moreMOREHUCH");
 }
 
 TEST(strextend_with_separator) {
@@ -285,6 +291,9 @@ TEST(strextend_with_separator) {
         ASSERT_STREQ(str, "start,,1,234");
         assert_se(strextend_with_separator(&str, ";", "more", "5", "678"));
         ASSERT_STREQ(str, "start,,1,234;more;5;678");
+
+        assert_se(strextend_with_separator(&str, ";", "xxxx", POINTER_MAX, "yyy"));
+        ASSERT_STREQ(str, "start,,1,234;more;5;678;xxxx;yyy");
 }
 
 TEST(strrep) {
@@ -400,6 +409,10 @@ TEST(strjoin) {
         actual = strjoin("foo", NULL, "bar");
         ASSERT_STREQ(actual, "foo");
         free(actual);
+
+        actual = strjoin("foo", POINTER_MAX, "bar");
+        ASSERT_STREQ(actual, "foobar");
+        free(actual);
 }
 
 TEST(strcmp_ptr) {