profiles: Generate an 'etc/profile' file.
Suggested by 宋文武 <iyzsong@gmail.com>
in <http://bugs.gnu.org/20255>.
* guix/build/profiles.scm (abstract-profile,
write-environment-variable-definition): New procedures.
(build-profile): Add #:search-paths parameter. Create
OUTPUT/etc/profile.
* guix/profiles.scm (profile-derivation)[builder]: Add 'search-paths'
variable and pass it to 'build-profile'. Adjust #:modules argument.
* tests/profiles.scm ("etc/profile"): New test.
* doc/guix.texi (Invoking guix package): Mention etc/profile.