gnu: Add perl-uri.

* gnu/packages/web.scm (perl-uri): New variable.

gnu: Add perl-http-date.

* gnu/packages/web.scm (perl-http-date): New variable.

gnu: Add perl-html-parser.

* gnu/packages/web.scm (perl-html-parser): New variable.

gnu: Add perl-html-tagset.

* gnu/packages/web.scm (perl-html-tagset): New variable.

Add httpd

* gnu/packages/web.scm (httpd): New variable
* gnu-system.am (GNU_SYSTEM_MODULES): Add it

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