shared/conf-parser: allow sections to be silently ignored with new -Section syntax

If we ignore any uknown section, we will not be able to show any
warning if a typo in a section name is made. Let's reverse our
approach, and explicitly list sections to ignore instead.

I opted to make use the same section list for this, instead of adding a second
list, because this list is passed through to many functions and adding yet
another parameter to the long signature would be very noisy.

src/shared/conf-parser.c

@@ -221,8 +221,19 @@ static int parse_line(
                         return -ENOMEM;
 
                 if (sections && !nulstr_contains(sections, n)) {
+                        bool ignore = flags & CONFIG_PARSE_RELAXED;
+                        const char *t;
 
-                        if (!(flags & CONFIG_PARSE_RELAXED) && !startswith(n, "X-"))
+                        ignore = ignore || startswith(n, "X-");
+
+                        if (!ignore)
+                                NULSTR_FOREACH(t, sections)
+                                        if (streq_ptr(n, startswith(t, "-"))) {
+                                                ignore = true;
+                                                break;
+                                        }
+
+                        if (!ignore)
                                 log_syntax(unit, LOG_WARNING, filename, line, 0, "Unknown section '%s'. Ignoring.", n);
 
                         free(n);