This commit makes the parent of XMEGA parts one of three common ones:
 - .xmega-ab for XMEGA-A or XMEGA-B parts
 - .xmega-cd for XMEGA-C or XMEGA-D parts
 - .xmega-e for  XMEGA-E parts

The commit does not change any feature of parts or memories.
In order to prove this carry out before this commit

$ avrdude -p */At >/tmp/1

After this commit, the following command should not have any output:

$ avrdude -p */At | sort | comm -3 - /tmp/1 | grep -v family.common.values

(The only feature changes in the .xmega-* prototypes.)

Compared the T WD_FUSE etc values in the data sheets for m169, m169a,
m169p, m169pa, m3290a, m3290p, m3290pa, m329a, m329p, m329pa, m6490a,
m6490p, m649a, m649p and m8535.

Although, on the surface of it, every entry of a classic part has been
changed, and some radically so by different parenting, there are only
subtle changes in AVRDUDE's internal representation:
 - 15 parts that did not have an io memory entry now have one
 - Some ISP opcodes were replaced with equivalent ones (x maps to 0)

This has been checked using the development options, particularly -p*/At,
before and after the changes.

The benefit of the new avrdude.conf is that every (non-TPI) classic part
is now derived from a common .classic or .classic-nocal stub. This enables
putting a common prodsig and sernum memory there. Generally, parenting
off now only happens between related chips. All in all, more systematic.
And it's some 10% smaller, too.