daemon: Use 'Agent' to spawn 'guix substitute --query'.

* nix/libstore/local-store.hh (RunningSubstituter): Remove.
(LocalStore)[runningSubstituter]: Change to unique_ptr<Agent>.
[setSubstituterEnv, didSetSubstituterEnv]: Remove.
[getLineFromSubstituter, getIntLineFromSubstituter]: Take an 'Agent'.
* nix/libstore/local-store.cc (LocalStore::~LocalStore): Remove
reference to 'runningSubstituter'.
(LocalStore::setSubstituterEnv, LocalStore::startSubstituter): Remove.
(LocalStore::getLineFromSubstituter): Adjust to 'run' being an 'Agent'.
(LocalStore::querySubstitutablePaths): Spawn substituter agent if
needed.  Adjust to 'Agent' interface.
(LocalStore::querySubstitutablePathInfos): Likewise.
* nix/libstore/build.cc (SubstitutionGoal::tryToRun): Remove call to
'setSubstituterEnv' and add 'setenv' call for "_NIX_OPTIONS" instead.
(SubstitutionGoal::finished): Remove 'readLine' call for 'dummy'.
* guix/scripts/substitute.scm (%allow-unauthenticated-substitutes?):
Remove second argument to 'make-parameter'.
(process-query): Call 'warn-about-missing-authentication'
when (%allow-unauthenticated-substitutes?) is #t.
(guix-substitute): Wrap body in 'parameterize'.  Set 'guix-warning-port'
too.  No longer exit when 'substitute-urls' returns the empty list.  No
longer print newline initially.
* tests/substitute.scm (test-quit): Parameterize 'current-error-port' to
account for the port changes in 'guix-substitute'.

tests/substitute.scm

@@ -47,7 +47,8 @@ it writes to GUIX-WARNING-PORT a messages that matches ERROR-RX."
   (test-equal name
     '(1 #t)
     (let ((error-output (open-output-string)))
-      (parameterize ((guix-warning-port error-output))
+      (parameterize ((current-error-port error-output)
+                     (guix-warning-port error-output))
         (catch 'quit
           (lambda ()
             exp