nft: Merge xtables-arp-standalone.c into xtables-standalone.c

nft: Merge xtables-arp-standalone.c into xtables-standalone.c

By declaring the relevant family_ops callbacks for arptables, the code
becomes ready to just use do_commandx() instead of a dedicated parser.

As a side-effect, this enables a bunch of new features in arptables-nft:

* Support '-C' command
* Support '-S' command
* Support rule indexes just like xtables, e.g. in '-I' or '-R' commands
* Reject chain names starting with '!'
* Support '-c N,M' counter syntax

Since arptables still accepts intrapositioned negations, add code to
cover that but print a warning like iptables did 12 years ago prior to
removing the functionality.

Signed-off-by: Phil Sutter <phil@nwl.cc>