comm: share standard input between both operands

[harshasiddartha]

printf 'test' | comm - - hangs forever without printing anything.

Each operand is opened on its own, and - becomes stdin().lock(). That lock is not reentrant, so the second - blocks on it before any input is read.

Both operands now share one lock, taking lines from the single stream in turn. That is what GNU comm prints too: for a\nb\nc\n on standard input, GNU and this branch both write a, \tb, c. GNU also reports comm: -: Bad file descriptor and exits 1, which comes from it closing standard input twice; that is not reproduced here, and the exit status is 0 as with BSD comm.

One difference I did not chase: with --total, GNU prints no totals line, because it dies in the double close before reaching it, while this branch prints the counts. That follows from the same double-close artifact described above.

Tested with a new case in tests/by-util/test_comm.rs that pipes a\nb\nc\n into comm - - and checks the output. It has an explicit timeout, so it fails instead of hanging if the deadlock ever comes back; it does fail on that timeout without this change.

Fixes #14219

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[oech3]

I don't want to have stdin specific code just for 1 undefined behaviour.
Note that this is actually applied for all 2 same streams e.g. a shared named fifo.