gnu: tcl: Install man pages in share/man.

* gnu/packages/tcl.scm (tcl): Add --mandir configure flag.
daemon: Change some options via 'settings.set'.

* nix/nix-daemon/guix-daemon.cc (parse_opt): Use 'settings.set' instead
  of direct field access for 'buildCores', 'maxBuildJobs', and
  'useSubstitutes'.
  (main): Call 'settings.update' after 'argp_parse'.
daemon: Clear $NIX_SUBSTITUTERS when passed '--no-substitutes'.

* nix/nix-daemon/guix-daemon.cc (main): When --no-substitutes is used,
  clear NIX_SUBSTITUTERS.  Before that, and after
  89faa5c75cb3a419c6d02c51f56955275b7ae351, '--no-substitutes' would
  lead to attempts to use 'download-using-manifests.pl', which in
  practice would gracelessly fail.
daemon: Add tests for substitutes and --no-substitutes.

* tests/guix-daemon.sh: Add substituter tests.
tests: Make sure 'guix archive --import' succeeds.

* tests/guix-archive.sh: Check the exit value of 'guix archive
  --import'.
gnu: ocrad: Upgrade to 0.23.

* gnu/packages/ocrad.scm (ocrad): Upgrade to 0.23.
gnu: ed: Upgrade to 1.10.

* gnu/packages/ed.scm (ed): Upgrade to 1.10.  Use '.tar.lz' file, and
  add 'native-inputs' field.