offload: Make the compression methods configurable.

* guix/scripts/offload.scm (<build-machine>)[compression]
[compression-level]: New fields.
(open-ssh-session): Honor them.
* doc/guix.texi (Daemon Offload Setup): Document them.

doc/guix.texi

@@ -921,6 +921,13 @@ Port number of SSH server on the machine.
 The SSH private key file to use when connecting to the machine, in
 OpenSSH format.
+@item @code{compression} (default: @code{"zlib@@openssh.com,zlib"})
+@itemx @code{compression-level} (default: @code{3})
+The SSH-level compression methods and compression level requested.
+
+Note that offloading relies on SSH compression to reduce bandwidth usage
+when transferring files to and from build machines.
+
 @item @code{daemon-socket} (default: @code{"/var/guix/daemon-socket/socket"})
 File name of the Unix-domain socket @command{guix-daemon} is listening
 to on that machine.

guix/scripts/offload.scm

@@ -75,6 +75,10 @@ (define-record-type* <build-machine>
   (private-key     build-machine-private-key      ; file name
                    (default (user-openssh-private-key)))
   (host-key        build-machine-host-key)        ; string
+  (compression     build-machine-compression  ; string
+                   (default "zlib@openssh.com,zlib"))
+  (compression-level build-machine-compression-level ;integer
+                     (default 3))
   (daemon-socket   build-machine-daemon-socket    ; string
                    (default "/var/guix/daemon-socket/socket"))
   (parallel-builds build-machine-parallel-builds  ; number
@@ -175,8 +179,10 @@ (define (open-ssh-session machine)
 
                                ;; We need lightweight compression when
                                ;; exchanging full archives.
-                               #:compression "zlib"
-                               #:compression-level 3)))
+                               #:compression
+                               (build-machine-compression machine)
+                               #:compression-level
+                               (build-machine-compression-level machine))))
     (match (connect! session)
       ('ok
        ;; Authenticate the server.  XXX: Guile-SSH 0.10.1 doesn't know about