profiles: Generate an 'etc/profile' file.

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.