enable gocritic linter

The linter ensures a common code style.
- use switch/case instead of else if
- use if instead of switch/case for single case statement
- add space between comment and text
- detect the use of defer with os.Exit()
- use short form var += "..." instead of var = var + "..."
- detect problems with append()
```
newSlice := append(orgSlice, val)
```
  This could lead to nasty bugs because the orgSlice will be changed in
  place if it has enough capacity too hold the new elements. Thus we
  newSlice might not be a copy.

Of course most of the changes are just cosmetic and do not cause any
logic errors but I think it is a good idea to enforce a common style.
This should help maintainability.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

libpod/container_api.go

@@ -229,8 +229,7 @@ func (c *Container) Kill(signal uint) error {
 // This function returns when the attach finishes. It does not hold the lock for
 // the duration of its runtime, only using it at the beginning to verify state.
 func (c *Container) Attach(streams *define.AttachStreams, keys string, resize <-chan define.TerminalSize) error {
-	switch c.LogDriver() {
-	case define.PassthroughLogging:
+	if c.LogDriver() == define.PassthroughLogging {
 		return errors.Wrapf(define.ErrNoLogs, "this container is using the 'passthrough' log driver, cannot attach")
 	}
 	if !c.batched {