gexp: computed-file: Prevent mistakenly overriding default option values.

In order to do so, default to an empty options list, and expose options whose
default values are sensitive directly as keyword arguments.

* guix/gexp.scm (computed-file): Extract the LOCAL-BUILD? parameter from the
OPTIONS parameter to make it a stand-alone keyword argument.  Introduce an
OPTIONS* binding which is obtained by combining the LOCAL-BUILD? keyword and
its value with OPTIONS.
* doc/guix.texi (G-Expressions): Adjust doc.

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