Add functionality for sending READDATA bodies

* doc/guile-curl.texi: remove broken menu, add READDATA option
* src/const.c (CURLOPT_READDATA): new global
* src/curl.scm.in (curl-easy-setopt): add readdata option
* src/func.c (read_callback): new procedure
  (cl_easy_setopt): add hook for CURLOPT_READDATA