[NETFILTER]: xt_sctp: simplify xt_sctp.h

The use of xt_sctp.h flagged up -Wshadow warnings in userspace, which
prompted me to look at it and clean it up. Basic operations have been
directly replaced by library calls (memcpy, memset is both available
in the kernel and userspace, and usually faster than a self-made
loop). The is_set and is_clear functions now use a processing time
shortcut, too.

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>