Fix failure to check for open() or fsync() failures.

Fix failure to check for open() or fsync() failures.

While it seems OK to not be concerned about fsync() failure for a
pre-existing signal file, it's not OK to not even check for open()
failure.  This at least causes complaints from static analyzers,
and I think on some platforms passing -1 to fsync() or close() might
trigger assertion-type failures.  Also add (void) casts to make clear
that we're ignoring fsync's result intentionally.

Oversights in commit 2dedf4d9a, noted by Coverity.