cl-easy-setopt processing for CURLOPT_POSTFIELDS incorrect

* src/func.c (cl_easy_setopt): stricter validation
* src/type.c (_scm_convert_to_byte_data): make sure to allocate fresh storage
* src/check_type.c (test_free_handle_string_postfields): new test
  (test_free_handle_bytevector_postfields): new test
Add basic handle tests

* test/check_type.c (test_equalp_handle): new test
  (test_not_equalp_handle, test_free_handle, test_not_free_handle): new tests
  (test_double_free_handle): new test
Set SMOB data to NULL on free

* src/type (gc_free_handle): modified
Add more paranoia in low-level handle functions

* src/type.c (equalp_handle): assert input types
  (print_handle): assert input types
  (_scm_convert_to_slist): add new error checks