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.
dd: new status=progress level to print stats periodically

* src/dd.c: Report the transfer progress every second when the
new status=progress level is used.  Adjust the handling and
description of the status= option so that they're treated as
mutually exclusive levels, rather than flags with implicit precedence.
* doc/coreutils.texi (dd invocation): Document the new progress
status level.  Reference the new level in the description of SIGUSR1.
* tests/dd/stats.sh: Add new test for status=progress.
* tests/dd/misc.sh: Change so status=none only takes precedence
if it's the last level specified.
* NEWS: Mention the feature.
maint: avoid double semicolon syntax check failure

A syntax-check recently added to gnulib would trigger a failure
(once gnulib gets updated here) for a statement introduced with
commit v8.23-43-gaf2a4ed:

  src/dd.c:806:  char const *time_fmt = _(", %g s, %s/s\n");;
  maint.mk: Double semicolon detected
  make: *** [sc_prohibit_double_semicolon] Error 1

* src/dd.c (print_xfer_stats): s/;;/;/
tests: fix false failure for test referencing libdl

* init.cfg (gcc_shared_): -ldl has to be positioned after the object
files that may rely upon it.  This fixes tests/cp/nfs-removal-race.sh
which references dlsym() from libdl.