dd: use more robust SIGUSR1 handling

* src/dd.c (ifd_reopen): A new wrapper to ensure we
don't exit upon receiving a SIGUSR1 in a blocking open()
on a fifo for example.
(iftruncate): Likewise for ftruncate().
(iread): Process signals also after a short read.
(install_signal_handlers): Install SIGINFO/SIGUSR1 handler
even if set to SIG_IGN, as this is what the parent can easily
set from a shell script that can send SIGUSR1 without the
possiblity of inadvertently killing the dd process.
* doc/coreutils.texi (dd invocation): Improve the example to
show robust usage wrt signal races and short reads.
* tests/dd/stats.sh: A new test for various signal races.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.

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