Add 'guix shell'.

* guix/scripts/shell.scm, tests/guix-shell.sh: New files.
* Makefile.am (MODULES): Add 'shell.scm'.
(SH_TESTS): Add 'tests/guix-shell.sh'.
* guix/scripts/environment.scm (show-environment-options-help): New
procedure.
(show-help): Use it.
(guix-environment*): New procedure.
(guix-environment): Use it.
* po/guix/POTFILES.in: Add it.
* doc/guix.texi (Features): Refer to "guix shell"
(Invoking guix package): Likewise.
(Development): Likewise.
(Invoking guix shell): New node.
(Invoking guix environment): Add deprecation warning.
(Debugging Build Failures): Use 'guix shell' in examples.
(Invoking guix container): Refer to 'guix shell'.
(Invoking guix processes, Virtualization Services): Adjust examples to
use 'guix shell'.
* doc/contributing.texi (Building from Git): Refer to 'guix shell'.
* etc/completion/bash/guix: Handle "shell".

tests/guix-shell.sh

@@ -0,0 +1,54 @@
+# GNU Guix --- Functional package management for GNU
+# Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
+#
+# This file is part of GNU Guix.
+#
+# GNU Guix is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or (at
+# your option) any later version.
+#
+# GNU Guix is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+#
+# Test the 'guix shell' alias.
+#
+
+guix shell --version
+
+tmpdir="t-guix-shell-$$"
+trap 'rm -r "$tmpdir"' EXIT
+mkdir "$tmpdir"
+
+guix shell --bootstrap --pure guile-bootstrap -- guile --version
+
+# '--ad-hoc' is a thing of the past.
+! guix shell --ad-hoc guile-bootstrap
+
+if guile -c '(getaddrinfo "www.gnu.org" "80" AI_NUMERICSERV)' 2> /dev/null
+then
+    # Compute the build environment for the initial GNU Make.
+    guix shell --bootstrap --no-substitutes --search-paths --pure \
+         -D -e '(@ (guix tests) gnu-make-for-tests)' > "$tmpdir/a"
+
+    # Make sure bootstrap binaries are in the profile.
+    profile=`grep "^export PATH" "$tmpdir/a" | sed -r 's|^.*="(.*)/bin"|\1|'`
+
+    # Make sure the bootstrap binaries are all listed where they belong.
+    grep -E "^export PATH=\"$profile/bin\""         "$tmpdir/a"
+    grep -E "^export CPATH=\"$profile/include\""    "$tmpdir/a"
+    grep -E "^export LIBRARY_PATH=\"$profile/lib\"" "$tmpdir/a"
+    for dep in bootstrap-binaries-0 gcc-bootstrap-0 glibc-bootstrap-0
+    do
+	guix gc --references "$profile" | grep "$dep"
+    done
+
+    # 'make-boot0' itself must not be listed.
+    ! guix gc --references "$profile" | grep make-boot0
+fi