MFC r313477:

MFC r313477:

Cleanup on usr.sbin/arp/arp.c

* 'blackhole' and 'reject' are mutually exclusive, replace printf() by errx()
  when both are selected.
* 'trail' option is no longer supported since first import of arp from 4.4BSD.
  XXX message was added 13 years ago in r128192. I believe it's time to remove
  it.
* Use warnx() to print some informative messages instead of printf()
* Replace strncmp() by strcmp() when validating parameters and exit when invalid
  parameter is found

Reviewed by:	allanjude, vangyzen, cem
Approved by:	allanjude
MFC after:	1 week
Sponsored by:	Rubicon Communications (Netgate)
Differential Revision:	https://reviews.freebsd.org/D9504