Package config: Add types for common configuration needs.

[octo]

The "config".Unmarshaler interface can be used to implement unmarshaling behavior that differs from the default. That can be used to support some recurring needs:

• A type Port uint16 that accepts numeric and string values. If a string, "net".LookupPort is used to convert it to an integer. If the numeric value is outside of the 1–65535 range, an error is returned.
• A type ServiceName string that is the inverse: a numeric value (in the 1–65535 range) is converted to a string.
• Control mapping of multiple values onto a scalar type. For example, when mapping ["foo", "bar", "qux"] to a single string, should this result in an error (current behavior), the first value ("foo"), the last value ("qux"), or a random value? Special types could handle that. The "last value wins" / value is overwritten behavior is not uncommon in C plugins.
• Provide minimum / maximum values for numeric values.
• Check string values with a regular expression.