gnu: conkeror: install conkeror-spawn-helper.

* gnu/packages/conkeror.scm (conkeror)[build-system]: Change to
  gnu-build-system.  Adjust arguments accordingly.
  [inputs]: Remove now-implicit bash.
  [native-inputs]: Remove now-implicit tar and gzip.
  [description]: Fix two-space-after-end-of-sentence.  Remove comment
  about separate conkeror-spawn-process-helper package.

gnu: Add conkeror.

* gnu-system.am (GNU_SYSTEM_MODULES): Add new package.
* gnu/packages/conkeror.scm: New file.

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