make a relay_header_t struct and pack/unpack funcs

split 7-byte stream_id string into 2-byte recognized and 2-byte stream_id
fix two seg faults in fetch_from_buf_http
fix several lurking seg faults in handling unexpected relay cells

still need to
 * clean up relay_crypt
 * use relay dummies if there's going to be a conflict with rh.recognized
 * check for a conflict when generating stream_ids


svn:r953