Keep the bytes a cancelled read took from the descriptor

[EdmondDantes]

Closes #288.

A file read is submitted with offset = -1, so read(2) moves the descriptor offset. A cancelled or timed-out coroutine returned -1 and disposed the request, and the bytes the worker had already taken went to nobody: the next reader on that handle carried on past them, and ftell() answered a number unrelated to the descriptor. On a file whose every 16 bytes carry their own record number, after a cancelled 64 KB read the next read returned the record at byte 65536 while ftell() said 32.

Three parts, two of them here:

• A read abandoned mid-flight seeks the descriptor back by what it took. The rewind waits for the thread pool to be empty: a write, an fsync or a sendfile on the same handle moves that offset too.
• File reads are serialized per handle the way file writes already are, so nothing reads from the advanced offset before the rewind. Without the queue the test is red 6 runs in 10; with it, 0 in 10. The two queues share one unlink helper — the copy it replaces cleared q_next while looking in the other queue, which cut the list in two.
• php-src side, true-async c9932d7d878: a read that finished before the exception arrived hands its count back instead of -1, since the bytes are already in the caller's buffer. A stream with a read filter keeps the old answer — the filter call under a pending exception returns without running, and the filter layer reads that as a fatal error the stream never recovers from.

A submit that fails now completes the request with its exception instead of throwing, which is what the write path already does: fread() reports the error as -1 with a notice rather than as an exception.

Verification

• tests/io/100-cancel_keeps_the_position.phpt: red 4 runs in 5 before, green 10 in 10 after.
• Debug: ext/async/tests plus php-src file, streams, filters, general_functions, zlib, phar, session — 3831 tests, 3 failures, all three the local symlink and network artefacts (curl 063, curl 064, io 082).
• ASAN: ext/async/tests plus php-src file, streams, filters — 1937 tests, 1 failure, the local network artefact.
• Cost: one read at a time per handle is what the stream's read-side lock already enforces, so the queue is only reached when a cancelled read is still in flight. 256 MB in 8 KB chunks on a release ZTS build, medians of five runs: 2226.3 ms before, 2171.6 ms after.

[codecov]

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