LibCore: Make IODevice::read_line() return a String

Almost everyone using this API actually wanted String instead of a
ByteBuffer anyway, and there were a bunch of slightly different ways
clients would convert to String.

Let's just cut out all the confusion and make it return String. :^)

Libraries/LibCore/ConfigFile.cpp

@@ -83,8 +83,8 @@ void ConfigFile::reparse()
     HashMap<String, String>* current_group = nullptr;
 
     while (file->can_read_line()) {
-        auto line = file->read_line(BUFSIZ);
-        auto* cp = (const char*)line.data();
+        auto line = file->read_line();
+        auto* cp = line.characters();
 
         while (*cp && (*cp == ' ' || *cp == '\t' || *cp == '\n'))
             ++cp;