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

Fixes #1463.

man/mu-extract.1.org

@@ -8,7 +8,7 @@
 
 * SYNOPSIS
 
-*mu [common-options] extract [options] <file>*
+*mu [common-options] extract [options] [<file>]*
 
 *mu [common-options] extract [options] <file> <pattern>*
 
@@ -30,6 +30,10 @@ Without any options, *mu extract* simply outputs the list of leaf MIME-parts in
 the message. Only 'leaf' MIME-parts (including RFC822 attachments) are
 considered, *multipart/** etc. are ignored.
 
+Without a filename parameter, ~mu extract~ reads a message from standard-input. In
+that case, you cannot use the second, ~<pattern>~ parameter as this would be
+ambiguous; instead, use the ~--matches~ option.
+
 * EXTRACT OPTIONS
 
 ** -a, --save-attachments
@@ -56,6 +60,11 @@ by default, ~mu~ transforms the attachment filenames a bit (such as by replacing
 spaces by dashes); with this option, leave that to the minimum for creating
 a legal filename in the target directory.
 
+** --matches=<pattern>
+Attachments with filenames matching the pattern will be extracted. The regular
+expressions are basic PCRE, and are case-sensitive by default; see *pcre(3)* for
+more details.
+
 ** --play
 Try to 'play' (open) the attachment with the default application for the
 particular file type. On MacOS, this uses the *open* program, on other platforms
@@ -87,6 +96,11 @@ To extract an mp3-file, and play it in the default mp3-playing application:
 $ mu extract --play msgfile 'whoopsididitagain.mp3'
 #+end_example
 
+when reading from standard-input, you need ~--matches~, so:
+#+begin_example
+$ cat msgfile | mu extract --play --matches 'whoopsididitagain.mp3'
+#+end_example
+
 #+include: "prefooter.inc" :minlevel 1
 
 * SEE ALSO

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

man/mu-view.1.org

@@ -7,7 +7,7 @@ mu view - display an e-mail message file
 
 * SYNOPSIS
 
-mu [common options] view [options] <file> [<files>]
+mu [common options] view [options] [<file> ...]
 
 * DESCRIPTION
 
@@ -17,6 +17,9 @@ message files and does =not= require the message to be indexed in the database.
 The command shows some common headers (From:, To:, Cc:, Bcc:, Subject: and
 Date:), the list of attachments and the plain-text body of the message (if any).
 
+If no message file is provided, the command expects the message on
+standard-input.
+
 * VIEW OPTIONS
 
 ** --summary-len=<number>