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