BUG #6651: macaddr length constraint violates standard

The following bug has been logged on the website:

Bug reference:      6651
Logged by:          Alex North-Keys
Email address:      erlkonig@talisman.org
PostgreSQL version: 9.1.3
Operating system:   any
Description:=20=20=20=20=20=20=20=20

The macaddr type does not allow for MACs of greater length (or less than)
than six bytes, only capturing a particular variety of ethernet address
(Xerox's original version) instead of the broader use of MACs where
addresses of other lengths are common, such as the 64-bit EUI-64.

A more appropriate type name would have been "ethernet", specific to the
case actually captured by "macaddr".  Given compatibility concerns, renaming
it is probably infeasible.  However, being able to parameterize the length,
defaulting to 6 bytes, would be adequate.