V2 Add support for ssh authentication methods

* podman --remote ssh://<user>:<password>@<host>:<port><path>
* podman --remote ssh://<user>:<password>@<host>:<port><path> \
  --identity <path> --passphrase <phrase>
* ssh-add <key>
  podman --remote ssh://<user>@<host><path>
* Fix `podman help` to run even if podman missing components
* Prompt for passphrase on stdin IFF key is protected and passphrase
  not given via any other configuration

* cobra flags do not support optional value flags therefore refactored
  --remote to be a boolean and --url will now contain the URI to Podman
  service

Signed-off-by: Jhon Honce <jhonce@redhat.com>

pkg/bindings/bindings.go

@@ -9,7 +9,13 @@
 package bindings
 
 import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+
 	"github.com/blang/semver"
+	"golang.org/x/crypto/ssh/terminal"
 )
 
 var (
@@ -25,3 +31,40 @@ var (
 	// _*YES*- podman will fail to run if this value is wrong
 	APIVersion = semver.MustParse("1.0.0")
 )
+
+// readPassword prompts for a secret and returns value input by user from stdin
+// Unlike terminal.ReadPassword(), $(echo $SECRET | podman...) is supported.
+// Additionally, all input after `<secret>/n` is queued to podman command.
+func readPassword(prompt string) (pw []byte, err error) {
+	fd := int(os.Stdin.Fd())
+	if terminal.IsTerminal(fd) {
+		fmt.Fprint(os.Stderr, prompt)
+		pw, err = terminal.ReadPassword(fd)
+		fmt.Fprintln(os.Stderr)
+		return
+	}
+
+	var b [1]byte
+	for {
+		n, err := os.Stdin.Read(b[:])
+		// terminal.ReadPassword discards any '\r', so we do the same
+		if n > 0 && b[0] != '\r' {
+			if b[0] == '\n' {
+				return pw, nil
+			}
+			pw = append(pw, b[0])
+			// limit size, so that a wrong input won't fill up the memory
+			if len(pw) > 1024 {
+				err = errors.New("password too long, 1024 byte limit")
+			}
+		}
+		if err != nil {
+			// terminal.ReadPassword accepts EOF-terminated passwords
+			// if non-empty, so we do the same
+			if err == io.EOF && len(pw) > 0 {
+				err = nil
+			}
+			return pw, err
+		}
+	}
+}