Include formatting check to the `make test` (and thus also `check`) rule (#1366)

Include formatting check to the `make test` (and thus also `check`) rule (#1366)

* Include formatting check to the `make check` rule

... and give it its own standalone target too (make fmt-check)

Show diff on fmt-check failure
Do not allow running "fmt-check" with incompatible go version
Also simplify the `fmt` rule

* Forbid running "make fmt" with Go version != 1.7 or 1.8

* Ignore bindata.go for spelling and formatting checks

also remove duplicated variable assignment for GOFILES

* Run `make fmt`