Fix a bug, add some functionalities to _buffer_ module.

- fix a bug regarding to the EOF parameter in `buffer_fill_fd()`
- add `buffer-truncate()` to truncate some contents to the end of the buffer
- `BUFNODE_ITER()` to iterate all bufnodes
- `REARPAD_SIZE()` is increased to 80 for the multipart boundary
- remove unused code blocks
Rudimentary support for POST and PUT requests

- add `prepare_recv_body()` to handle `100-continue`
- fix a infite loop in `str2method()`
- fix a bug that can't handle a req. with no TE in `load_req_line_headers()`
- fix a bug in `ev_httpconn_io_cb()` when `buffer_seek()` failed
add some documentation for myself
add _form_ module, for `multipart/form-data` handling