misc/emacs: Add compatibility for GNU Emacs 23 and XEmacs >=21.5.32

This CL adds compatibility for GNU Emacs 23 (fixing fontification
issues) and XEmacs >=21.5.32 (fixing a lot of issues). Earlier
versions of XEmacs will not be supported because they do not
support POSIX character classes. Because of that, we also make use
of a lot of functions that were added in 21.5.32.

A known and currently unfixable issue with XEmacs is that go-mode
will not always fontify identifiers that use unicode correctly.

All changes for XEmacs are annotated in the diff.

Note: go--position-bytes is not currently used anywhere, but will
be in a future CL.

Fixes #4927.

R=golang-dev, adonovan, cw, patrick.allen.higgins, sameer
CC=golang-dev
https://golang.org/cl/7456051