mu extract/view/verify: allow reading message from stdin

Fixes #1463.

man/mu-verify.1.org

@@ -7,7 +7,7 @@ mu verify - verify message signatures and display information about them
 
 * SYNOPSIS
 
-*mu [common-options] verify [options] <msgfile>*
+*mu [common-options] verify [options] [<file> ... ]*
 
 * DESCRIPTION
 
@@ -15,6 +15,9 @@ mu verify - verify message signatures and display information about them
 signatures) and displaying information about them. The sub-command works on
 message files, and does not require the message to be indexed in the database.
 
+If no message file is provided, the command expects the message on
+standard-input.
+
 * VERIFY OPTIONS
 
 ** -r, --auto-retrieve