Use got_pkt_readlen() to parse pkt-lens.

Rewrite got-fetch-http I/O using bufio API. This should help make the code more portable supports almost everything we need out of the box. pledge needs the rpath promise because of libtls. This was technically also the case with the old API but we didn't actually enable pledge thanks to DEBUG_HTTP. The synchronous bufio wrappers should probably move to lib/bufio.c in the future. ok and feedback from op@

Rename got-http helper to got-fetch-http to match naming scheme. We aren't planning to support sending anyway. ok stsp@