client: clear suid/sgid bits on non-zero write

client: clear suid/sgid bits on non-zero write

According to [1], these bits should be cleared regardless of any exe bits on
the file. Also, add the required non-zero write check.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/pwrite.html

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>