Warn about IP addresses in hostname fields.

A common error is to specify an IP address as the result of an MX
lookup. The RFCs (974, 1035, 2181...) state that the result should be
a "domain name," which in this context, means "host name." This commit
adds a check on "hostname" fields that rejects a dotted-quad contained
therein. In particular, it catches the aforementioned MX record error.