gzip: fix a data-loss bug when decompressing with --suffix=''

gzip: fix a data-loss bug when decompressing with --suffix=''

* gzip.c (main): Disallow an empty --suffix=S also with -d.
Otherwise, "gzip -d -S '' F.gz" would ask if it's ok to remove the
existing file, "F.gz";  if you reply "yes", you'd lose all of that data.
Use of an empty suffix was already rejected in compression mode.
* gzip.1 (--suffix (-S)): Do not recommend to use "gunzip -S '' *".
Describe how the suffix is used when decompressing, too.
* NEWS (Bug fixes): mention the fix.
Reported by Ripduman Sohan.