Add 'guix-daemon.service' file for systemd.

* etc/guix-daemon.service.in: New file, provided by Ricardo Wurmus.
* daemon.am (CLEANFILES): Add etc/guix-daemon.service.
  (systemdservicedir, nodist_systemdservice_DATA): New variables.
  (EXTRA_DIST): Add etc/guix-daemon.service.in.
* doc/guix.texi (Binary Installation, Build Environment Setup): Mention
  'guix-daemon.service'.

.gitignore

@@ -125,3 +125,4 @@ GTAGS
 /doc/os-config-bare-bones.texi
 /doc/os-config-desktop.texi
 /doc/*.1
+/etc/guix-daemon.service

daemon.am

@@ -21,7 +21,7 @@
 #
 
 BUILT_SOURCES += nix/libstore/schema.sql.hh
-CLEANFILES += $(BUILT_SOURCES)
+CLEANFILES += $(BUILT_SOURCES) etc/guix-daemon.service
 
 noinst_LIBRARIES = libformat.a libutil.a libstore.a
 
@@ -181,10 +181,21 @@ endif BUILD_DAEMON_OFFLOAD
 nodist_libexec_SCRIPTS =			\
   nix/scripts/guix-authenticate
 
+# The '.service' file for systemd.
+systemdservicedir = $(libdir)/systemd/system
+nodist_systemdservice_DATA = etc/guix-daemon.service
+
+etc/guix-daemon.service: etc/guix-daemon.service.in	\
+			 $(top_builddir)/config.status
+	$(SED) -e 's|@''bindir''@|$(bindir)|' <		\
+	       "etc/guix-daemon.service.in" > "$@.tmp"
+	mv "$@.tmp" "$@"
+
 EXTRA_DIST +=					\
   nix/libstore/schema.sql			\
   nix/AUTHORS					\
-  nix/COPYING
+  nix/COPYING					\
+  etc/guix-daemon.service.in
 
 AM_TESTS_ENVIRONMENT +=				\
   top_builddir="$(abs_top_builddir)"

doc/guix.texi

@@ -330,6 +330,10 @@ Run the daemon:
 # ~root/.guix-profile/bin/guix-daemon --build-users-group=guixbuild
 @end example
 
+On hosts using the systemd init system, drop
+@file{~root/.guix-profile/lib/systemd/system/guix-daemon.service} in
+@file{/etc/systemd/system}.
+
 @item
 Make the @command{guix} command available to other users on the machine,
 for instance with:
@@ -512,8 +516,12 @@ Bash syntax and the @code{shadow} commands):
 @noindent
 The number of build users determines how many build jobs may run in
 parallel, as specified by the @option{--max-jobs} option
-(@pxref{Invoking guix-daemon, @option{--max-jobs}}).
-The @code{guix-daemon} program may then be run as @code{root} with:
+(@pxref{Invoking guix-daemon, @option{--max-jobs}}).  The
+@code{guix-daemon} program may then be run as @code{root} with the
+following command@footnote{If your machine uses the systemd init system,
+dropping the @file{@var{prefix}/lib/systemd/system/guix-daemon.service}
+file in @file{/etc/systemd/system} will ensure that
+@command{guix-daemon} is automatically started.}:
 
 @example
 # guix-daemon --build-users-group=guixbuild

etc/guix-daemon.service.in

@@ -0,0 +1,15 @@
+# This is a "service unit file" for the systemd init system to launch
+# 'guix-daemon'.  Drop it in /etc/systemd/system or similar to have
+# 'guix-daemon' automatically started.
+
+[Unit]
+Description=Build daemon for GNU Guix
+
+[Service]
+ExecStart=@bindir@/guix-daemon --build-users-group=guixbuild
+RemainAfterExit=yes
+StandardOutput=syslog
+StandardError=syslog
+
+[Install]
+WantedBy=multi-user.target