string-util: add new helper for finding line starting with specific string in a text buffer

We have implemented this manually a couple of times, and always wrong.
Hence let's implement this correctly for once and use everywhere.

src/basic/string-util.c

@@ -1260,3 +1260,26 @@ char *strdupcspn(const char *a, const char *reject) {
 
         return strndup(a, strcspn(a, reject));
 }
+
+char *find_line_startswith(const char *haystack, const char *needle) {
+        char *p;
+
+        assert(haystack);
+        assert(needle);
+
+        /* Finds the first line in 'haystack' that starts with the specified string. Returns a pointer to the
+         * first character after it */
+
+        p = strstr(haystack, needle);
+        if (!p)
+                return NULL;
+
+        if (p > haystack)
+                while (p[-1] != '\n') {
+                        p = strstr(p + 1, needle);
+                        if (!p)
+                                return NULL;
+                }
+
+        return p + strlen(needle);
+}