dd: be more careful about signal handling

dd: be more careful about signal handling

Problem reported by Hans Henrik Bergan (Bug#36007).
* NEWS: Mention this.
* src/dd.c (iclose, ifdatasync, ifstat, ifsync):
New functions, which are more careful about SIGINT.
(cleanup): Use iclose instead of close.
(finish_up): Process signals first.
(skip, dd_copy, main): Use ifstat instead of fstat.
(dd_copy): Use ifdatasync and ifsync instead of fdatasync and fsync.