http3: improvements across backends

- ngtcp2: using bufq for recv stream data
- internal stream_ctx instead of `struct HTTP` members
  for quiche, ngtcp2 and msh3
- no more QUIC related members in `struct HTTP`
- experimental use of recvmmsg(), disabled by default
  - testing on my old debian box shows no throughput improvements.
  - leaving it in, but disabled, for future revisit
- vquic: common UDP receive code for ngtcp2 and quiche
- vquic: common UDP send code for ngtcp2 and quiche
- added pytest skips for known msh3 failures
- fix unit2601 to survive torture testing
- quiche: using latest `master` from quiche and enabling large download
  tests, now that key change is supported
- fixing test_07_21 where retry handling of starting a stream
  was faulty
- msh3: use bufq for recv buffering headers and data
- msh3: replace fprintf debug logging with LOG_CF where possible
- msh3: force QUIC expire timers on recv/send to have more than
  1 request per second served

Closes #10772

lib/bufq.h

@@ -245,6 +245,28 @@ typedef ssize_t Curl_bufq_reader(void *reader_ctx,
 ssize_t Curl_bufq_slurp(struct bufq *q, Curl_bufq_reader *reader,
                         void *reader_ctx, CURLcode *err);
 
+/**
+ * Read up to `max_len` bytes and append it to the end of the buffer queue.
+ * if `max_len` is 0, no limit is imposed and the call behaves exactly
+ * the same as `Curl_bufq_slurp()`.
+ * Returns the total amount of buf read (may be 0) or -1 on other
+ * reader errors.
+ * Note that even in case of a -1 chunks may have been read and
+ * the buffer queue will have different length than before.
+ */
+ssize_t Curl_bufq_slurpn(struct bufq *q, size_t max_len,
+                         Curl_bufq_reader *reader, void *reader_ctx,
+                         CURLcode *err);
+
+/**
+ * Read *once* up to `max_len` bytes and append it to the buffer.
+ * if `max_len` is 0, no limit is imposed besides the chunk space.
+ * Returns the total amount of buf read (may be 0) or -1 on other
+ * reader errors.
+ */
+ssize_t Curl_bufq_sipn(struct bufq *q, size_t max_len,
+                       Curl_bufq_reader *reader, void *reader_ctx,
+                       CURLcode *err);
 
 /**
  * Write buf to the end of the buffer queue.