Add missing single quotation and backslash marks.

Change dot and underscore character keyword type.
"_" is a predeclared identifier, not a operator.
"." is a selector, x.f should be one identifier highlight.
So the fix is to change it.

Fixes #5775.
Fixes #5788.
Fixes #5798.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10480044