ExecCmd function refactor

[tedteng]

is every occurence really fixed now?

func ExecCmd(input []byte, cmd string, suppressedOutput bool, environment ...string) (err error) {
	var command *exec.Cmd
	parts := strings.Fields(cmd)
	head := parts[0]
	if len(parts) > 1 {
		parts = parts[1:]
	} else {
		parts = nil
	}
	command = exec.Command(head, parts...)
	...

the ExecCmd function should not be called with a cmd string and instead the caller should pass the name and arguments directly for exec.Command, otherwise an attacker could sneak in arguments

Originally posted by @petersutter in #267 (comment)