guix import pypi: add the Apache License 2.0

* guix/import/pypi.scm (string->license): add ASL2.0.

refresh: Add end-of-line to error message.

* guix/scripts/refresh.scm (%options)[select]: Add end-of-line to
  error message.

gnu: Add 'version-prefix' and 'version-major+minor'; use them.

The procedure version-prefix truncates a version string to a certain
number of components. version-major+minor is a special case of this,
which extracts the "major.minor" part of the string. Previously this
was handled by an expression common to several packages.

* guix/utils.scm (version-prefix, version-major+minor): New procedures
* gnu/packages/backup.scm (duplicity): Use version-major+minor.
* gnu/packages/cmake.scm (cmake): Likewise.
* gnu/packages/gnome.scm (libbonobo, libbonoboui): Likewise.
* gnu/packages/gnutls.scm (gnutls): Likewise.
* gnu/packages/mail.scm (gmime): Likewise.
* gnu/packages/mpd.scm (mpd): Likewise.
* gnu/packages/mpi.scm (openmpi): Likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

download: Add "Accept: */*" to the headers.

Fixes downloads from https://alioth.debian.org.
Reported by John Darrington <jmd@gnu.org>.

* guix/build/download.scm (http-fetch)[headers]: Add 'Accept'.

packages: Add 'package-transitive-supported-systems'.

* guix/packages.scm (package-transitive-supported-systems): New procedure.
* tests/packages.scm ("package-transitive-supported-systems"): New test.
* build-aux/hydra/gnu-system.scm (package->job): Use it.

derivations: 'graft-derivation' correctly handles multiple-output sources.

* guix/derivations.scm (graft-derivation): Add DRV as input for each one
  of OUTPUT-NAMES.

packages: Improve the package-to-derivation cache.

* guix/packages.scm (cache): Preserve the former values associated with
  PACKAGE.
  (cached): Bind SYSTEM in 'let', to prevent double evaluation.

derivations: Add #:system parameter to 'graft-derivation'.

* guix/derivations.scm (graft-derivation): Add #:system parameter.

derivations: Introduce 'graft' record type.

* guix/derivations.scm (<graft>): New record type.
  (graft-derivation): Rename 'replacements' to 'grafts', and expect it
  to be a list of <graft> records.  Adjust accordingly.
* tests/derivations.scm ("graft-derivation"): Use 'graft' instead of
  pairs in argument to 'graft-derivation'.

records: Improve the docstring of 'define-record-type*'.

* guix/records.scm (define-record-type*): Agument the docstring, give
  examples.

download: Allow use of substitutes.

See <https://bugs.gnu.org/18747> for the original report.

* guix/download.scm (url-fetch): Comment out #:local-build? argument.
* guix/git-download.scm (git-fetch): Likewise.
* guix/svn-download.scm (svn-fetch): Likewise.

guix lint: add the --checkers option.

* guix/scripts/lint.scm: add the "--checkers" option.
* doc/guix.texi (Invoking guix lint): Document it.
* tests/guix-lint.sh: New file
* Makefile.am (SCM_TESTS): Add it.

utils: Micro-optimize 'bytevector->base16-string'.

This reduces run time of (package-derivation s emacs) by ~1.5%.

* guix/utils.scm (bytevector->base16-string): Traverse BV from the end
  so as to use 'string-concatenate' instead of
  'string-concatenate-reverse'.

derivations: Micro-optimize 'imported-modules'.

This reduces run time of (package-derivation s emacs) by ~5%.

* guix/derivations.scm (search-path*): New procedure.
  (imported-modules): Use it instead of 'search-path'.

derivations: Memoize 'derivation-path->base16-hash'.

This reduces run time of (package-derivation s emacs) by ~10%.

* guix/derivations.scm (derivation-path->base16-hash): New procedure.
  (derivation-hash): Use it.

derivations: Add #:guile parameter to 'graft-derivation'.

* guix/derivations.scm (graft-derivation): Add #:guile parameter; pass
  it to 'build-expression->derivation'.

guix package: Add '--switch-generation' option.

* guix/scripts/package.scm (switch-to-generation): New procedure.
  (switch-to-previous-generation): Use it.
  (guix-package): Adjust for '--switch-generation' option.
* tests/guix-package.sh: Test it.
* doc/guix.texi (Invoking guix package): Document it.

profiles: Add 'relative-generation'.

* guix/profiles.scm: (relative-generation): New procedure.
  (previous-generation-number): Use it.

profiles: Add condition types for profiles and generations.

Suggested by Ludovic Courtès.

* guix/profiles.scm (&profile-error, &profile-not-found-error,
  &missing-generation-error): New condition types.
* guix/ui.scm (call-with-error-handling): Handle new types.
* guix/scripts/package.scm (roll-back, guix-package): Raise
  '&profile-not-found-error' where needed.

scripts: Add 'environment' command.

* guix/scripts/environment.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi ("Invoking guix environment"): New node.