Gitweb - provide site headers and footers

Gitweb - provide site headers and footers

This allows web sites with a header and footer standard for each page
to add them to the pages produced by gitweb.

Two new variables $site_header and $site_footer are defined (default
to null) each of which can specify a file containing the header and
footer html.

In addition, if the $stylesheet variable is undefined, a new array
@stylesheets (which defaults to a single element of gitweb.css) can be
used to specify more than one style sheet.  This allows the clasical
gitweb.css styles to be retained, but a site wide style sheet used
within the header and footer areas.

Signed-off-by: Alan Chandler <redacted>
Signed-off-by: Junio C Hamano <redacted>