Currently, ftdetect/gofiletype.vim set fileencodings to open the file as
utf-8 encoding event if the file does not contain multibyte characters.
But fileencodings is global option.

$ vim foo.txt
:set fileencodings
utf-8,ucs-bom,cp932

$ vim foo.go
:set fileencodings
utf-8

This change restore fileencodings before opening the file.
Also added specify fileformats=unix.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5718045