* gnu/services/base.scm (guix-machines-files-installation): New
procedure.
(<guix-configuration>)[build-machines]: New field.
(guix-activation): Call ‘ guix-machines-files-installation’.
(<guix-extension>)[build-machines]: New field.
(guix-extension-merge): Handle it.
(guix-service-type)[extend]: Likewise.
* doc/guix.texi (Daemon Offload Setup): Add note linking to
‘guix-configuration’.
(Base Services): Document ‘build-machines’ field of <guix-configuration>
and of <guix-extension>.
(Virtualization Services): Add ‘hurd-vm’ anchor.
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