daemon: Default to '--cores=0'.

Suggested by Deck Pickard <deck.r.pickard@gmail.com>.

* nix/nix-daemon/guix-daemon.cc (main): Set 'settings.buildCores' to 0.
* doc/guix.texi (Invoking guix-daemon): Adjust accordingly.

doc/guix.texi

@@ -586,7 +586,7 @@ Cache build failures.  By default, only successful builds are cached.
 Use @var{n} CPU cores to build each derivation; @code{0} means as many
 as available.
 
-The default value is @code{1}, but it may be overridden by clients, such
+The default value is @code{0}, but it may be overridden by clients, such
 as the @code{--cores} option of @command{guix build} (@pxref{Invoking
 guix build}).
 

nix/nix-daemon/guix-daemon.cc

@@ -264,6 +264,9 @@ main (int argc, char *argv[])
   /* Turn automatic deduplication on by default.  */
   settings.autoOptimiseStore = true;
 
+  /* Default to using as many cores as possible.  */
+  settings.buildCores = 0;
+
   argvSaved = argv;
 
   try