cmd/ethkey: new command line tool for keys (#15438)

ethkey is a new tool that serves as a command line interface to
the basic key management functionalities of geth. It currently
supports:
 
 - generating keyfiles
 - inspecting keyfiles (print public and private key)
 - signing messages
 - verifying signed messages

cmd/ethkey/README.md

@@ -0,0 +1,41 @@
+ethkey
+======
+
+ethkey is a simple command-line tool for working with Ethereum keyfiles.
+
+
+# Usage
+
+### `ethkey generate`
+
+Generate a new keyfile.
+If you want to use an existing private key to use in the keyfile, it can be 
+specified by setting `--privatekey` with the location of the file containing the 
+private key.
+
+
+### `ethkey inspect <keyfile>`
+
+Print various information about the keyfile.
+Private key information can be printed by using the `--private` flag;
+make sure to use this feature with great caution!
+
+
+### `ethkey sign <keyfile> <message/file>`
+
+Sign the message with a keyfile.
+It is possible to refer to a file containing the message.
+
+
+### `ethkey verify <address> <signature> <message/file>`
+
+Verify the signature of the message.
+It is possible to refer to a file containing the message.
+
+
+## Passphrases
+
+For every command that uses a keyfile, you will be prompted to provide the 
+passphrase for decrypting the keyfile.  To avoid this message, it is possible
+to pass the passphrase by using the `--passphrase` flag pointing to a file that
+contains the passphrase.