The DISPATCH and CALLFN macro definitions depend on an inconsistency
between the internal cpp mini-implementation and the language proper in
whether center-dot is an identifier character. The macro depends on it not
being an identifier character, but the resulting code depends on it being one.

Remove the dependence on the inconsistency by placing the center-dot into
the macro invocation rather that the body.

No semantic change. This is just renaming macro arguments.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/119320043