archive: Add '--missing'.

* guix/scripts/archive.scm (show-help, %options): Add '--missing'.
  (guix-archive)[lines]: New procedure.
  Use it to honor '--missing'.
* tests/guix-archive.sh: Add tests.
* doc/guix.texi (Invoking guix archive): Document '--missing'.

doc/guix.texi

@@ -938,6 +938,12 @@ package to a machine connected over SSH, one would run:
 guix archive --export emacs | ssh the-machine guix archive --import
 @end example
 
+@noindent
+However, note that, in this example, all of @code{emacs} and its
+dependencies are transferred, regardless of what is already available in
+the target machine's store.  The @code{--missing} option can help figure
+out which items are missing from the target's store.
+
 Archives are stored in the ``Nix archive'' or ``Nar'' format, which is
 comparable in spirit to `tar'.  When exporting, the daemon digitally
 signs the contents of the archive, and that digital signature is
@@ -959,6 +965,11 @@ therein into the store.  Abort if the archive has an invalid digital
 signature, or if it is signed by a public key not among the authorized
 keys (see @code{--authorize} below.)
 
+@item --missing
+Read a list of store file names from the standard input, one per line,
+and write on the standard output the subset of these files missing from
+the store.
+
 @item --generate-key[=@var{parameters}]
 @cindex signing, archives
 Generate a new key pair for the daemons.  This is a prerequisite before