1. Don't run long steps for Go versions other than 1.9 & 1.10
according to Golang Release Policy (two latest versions).
2. Switch to codecov.io, collect coverage only on Go 1.10 which
has fixes for multi-module coverage & ./... ignoring vendor.
3. Simplify Makefile.
With previous version, `go install` automatically picks up
package `man` and installs `gen.go` as `main` to the $GOPATH/bin which
is not what is expected. Move man page generator to separate
private folder.
Andrey Smirnov