altsvc: Fix building with cookies disables

ALTSVC requires Curl_get_line which is defined in lib/cookie.c inside a #if
check of HTTP and COOKIES. That makes Curl_get_line undefined if COOKIES is
disabled. Fix by splitting out the function into a separate file which can
be included where needed.

Closes #3717
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>

lib/curl_get_line.c

@@ -0,0 +1,55 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+
+#include "curl_setup.h"
+
+#include "curl_get_line.h"
+#include "curl_memory.h"
+/* The last #include file should be: */
+#include "memdebug.h"
+
+/*
+ * get_line() makes sure to only return complete whole lines that fit in 'len'
+ * bytes and end with a newline.
+ */
+char *Curl_get_line(char *buf, int len, FILE *input)
+{
+  bool partial = FALSE;
+  while(1) {
+    char *b = fgets(buf, len, input);
+    if(b) {
+      size_t rlen = strlen(b);
+      if(rlen && (b[rlen-1] == '\n')) {
+        if(partial) {
+          partial = FALSE;
+          continue;
+        }
+        return b;
+      }
+      /* read a partial, discard the next piece that ends with newline */
+      partial = TRUE;
+    }
+    else
+      break;
+  }
+  return NULL;
+}