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>