xshared: Share a common add_command() implementation

xshared: Share a common add_command() implementation

The shared definition of cmdflags is a super set of the previous one in
xtables-arp.c so while not being identical, they're compatible.

Avoid accidental array overstep in cmd2char() by incrementing an index
variable and checking its final value before using it as such.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>