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.h

@@ -253,3 +253,5 @@ size_t strspn_from_end(const char *str, const char *accept);
 
 char *strdupspn(const char *a, const char *accept);
 char *strdupcspn(const char *a, const char *reject);
+
+char *find_line_startswith(const char *haystack, const char *needle);