Fixes a bug whereby services indirectly depended on would not be
automatically instantiated.

* gnu/services.scm (instantiate-missing-services): Loop back when the
length of ADJUSTED is greater than that of INSTANCES.
* tests/services.scm ("instantiate-missing-services, indirect"): New test.

* gnu/services.scm (compute-boot-script): Rename 'mexps' to 'gexps' and
remove 'mlet' form.
(boot-service-type): Update comment.
(cleanup-gexp): Remove 'with-monad' and 'return'.
(activation-script): Rewrite in non-monadic style: use 'scheme-file'
instead of 'gexp->file'.
(gexps->activation-gexp): Remove 'mlet', return a gexp.
* gnu/services/shepherd.scm (shepherd-boot-gexp): Remove 'with-monad'
and 'return'.
* gnu/system.scm (operating-system-boot-script): Remove outdated comment.
* gnu/tests/base.scm (%cleanup-os): For 'dirty-service', remove
'with-monad' and 'return'.

* gnu/services.scm (system-service-type, boot-service-type)
(cleanup-service-type, activation-service-type)
(special-files-service-type, etc-service-type)
(setuid-program-service-type, profile-service-type)
(firmware-service-type, gc-root-service-type): Add 'description' field.

Fixes <https://bugs.gnu.org/26353>.
Reported by Danny Milosavljevic <dannym@scratchpost.org>.

* gnu/services.scm (cleanup-gexp): Add 'setenv' and 'setlocale' calls
before 'delete-file-recursively'.
* gnu/tests/base.scm (%cleanup-os, %test-cleanup): New variables.
(run-cleanup-test): New procedure.

* gnu/services.scm (compute-boot-script): Reverse MEXPS.
* gnu/system.scm (essential-services): Reverse order of
%SHEPHERD-ROOT-SERVICE, %ACTIVATION-SERVICE, and CLEANUP-SERVICE-TYPE.

This reduces the closure of (guix discovery) from 28 to 8 modules.

* guix/discovery.scm (scheme-files): Use 'format' instead of 'warning'.
(scheme-modules): Add #:warn parameter.  Use it instead of
'warn-about-load-error'.
(fold-modules): Add #:warn and pass it to 'scheme-modules'.
(all-modules): Likewise.
* gnu/bootloader.scm (bootloader-modules): Pass #:warn to 'all-modules'.
* gnu/packages.scm (fold-packages): Likewise.
* gnu/services.scm (all-service-modules): Likewise.
* guix/upstream.scm (importer-modules): Likewise.

* gnu/services.scm (boot-service-type) <compose>: Use the "identity"
  procedure instead of the "append" procedure because it more accurately
  reflects the intent, which is to simply return the single list of
  extensions to which fold-services applies the "compose" procedure.
  (activation-service-type) <compose>: Likewise.

This simplifies OS configuration: users no longer need to be aware of
what a given service depends on.

See the discussion at
<https://lists.gnu.org/archive/html/guix-devel/2018-01/msg00114.html>.

* gnu/services.scm (missing-target-error): New procedure.
(service-back-edges): Use it.
(instantiate-missing-services): New procedure.
* gnu/system.scm (operating-system-services): Call
'instantiate-missing-services'.
* tests/services.scm ("instantiate-missing-services")
("instantiate-missing-services, no default value"): New tests.
* gnu/services/version-control.scm (cgit-service-type)[extensions]: Add
FCGIWRAP-SERVICE-TYPE.
* gnu/tests/version-control.scm (%cgit-os): Remove NGINX-SERVICE-TYPE
and FCGIWRAP-SERVICE-TYPE instances.
* doc/guix.texi (Log Rotation): Remove 'mcron-service-type' in example.
(Miscellaneous Services): Remove 'nginx-service-type' and
'fcgiwrap-service-type' in Cgit example.

* gnu/services.scm (cleanup-gexp): Remove "/run/udev/watch.old" directory.

* gnu/services.scm (lookup-service-types): New procedure.
* tests/services.scm ("lookup-service-types"): New test.