gzip: do not store timestamp in gzip header

gzip: do not store timestamp in gzip header

Storing the original file's modification time in the output file is
harmful (precludes deterministic results) and unlike official gzip,
the busybox version provides no way to suppress this behavior; the -n
option is silently ignored. Rather than trying to make -n work, this
patch just removes the timestamp-storing functionality. It should be
considered deprecated anyway; it's not Y2038-safe and gunzip ignores
it by default.

Per RFC 1952, 0 is the correct value to store to indicate that there
is no timestamp.

Signed-off-by: Rich Felker <dalias@libc.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>