Add footnote about emptying httpheader

* doc/guile-curl.text: modified
Quiet doc warnings about missing periods after @xref

* doc/guile-curl.texi: modified
Add test for freeing slist options

* test/check_type.c (test_free_handle_slist_option): new test
  (test_free_handle_empty_slist_option): new test
  (test_free_handle_empty_httpheader, test_free_handle_httpheader): removed
_scm_convert_to_slist now returns NULL on bad input

* src/type.c (_scm_convert_to_slist): return NULL on bad input
Add tests for cl_is_handle_p

* test/check_type.c (test_cl_is_handle_p__handle): new test
  (test_cl_is_handle_p__true): new test
Add tests for _scm_can_convert_to_slist and _scm_convert_to_slist

* test/check_type.c (test_can_convert_to_slist__slist): new test
  (test_can_convert_to_slist__empty_list): new test
  (test_can_convert_to_slist__list_of_integers): new test
  (test_scm_convert_to_slist__slist, test_scm_convert_to_slist__empty_list)
  (test_convert_to_slist__list_of_integers): new tests