Switch to bufio Reader for exec streams

There were many situations that made exec act funky with input. pipes didn't work as expected, as well as sending input before the shell opened.
Thinking about it, it seemed as though the issues were because of how os.Stdin buffers (it doesn't). Dropping this input had some weird consequences.
Instead, read from os.Stdin as bufio.Reader, allowing the input to buffer before passing it to the container.

Signed-off-by: Peter Hunt <pehunt@redhat.com>

libpod/container_api.go

@@ -1,6 +1,7 @@
 package libpod
 
 import (
+	"bufio"
 	"context"
 	"io"
 	"io/ioutil"
@@ -361,7 +362,7 @@ type AttachStreams struct {
 	// ErrorStream will be attached to container's STDERR
 	ErrorStream io.WriteCloser
 	// InputStream will be attached to container's STDIN
-	InputStream io.Reader
+	InputStream *bufio.Reader
 	// AttachOutput is whether to attach to STDOUT
 	// If false, stdout will not be attached
 	AttachOutput bool