[mod_ssi] config ssi.conditional-requests

[mod_ssi] config ssi.conditional-requests

Summary:
A new SSI directive, "ssi.conditional-requests", allows to inform
lighttpd which SSI pages should be considered as cacheable and which
should not. In particular, the "ETag" & "Last-Modified" headers will
only be sent for those SSI pages for which the directive is enabled.

Long description:
"ETag" and "Last-Modified" headers were being sent for all SSI pages,
regardless of whether they were cacheable or not. And yet, there was
no cache validation at all for any SSI page.
This commit fixes these two minor issues by adding a new directive,
"ssi.conditional-requests", which allows to specify which SSI pages
are cacheable and which are not, and by adding cache validation to
those SSI pages which are cacheable. And since sending ETags for
non-cacheable documents is not appropriate, they are no longuer
computed nor sent for those SSI pages which are not cacheable.
Regarding the "Last-Modified" header for non-cacheable documents,
the standards allow to either send the current date and time for
that header or to simply skip it. The approach chosen is to not send
it for non-cacheable SSI pages. "ETag" and "Last-Modified" headers
are therefore only sent for an SSI page if ssi.conditional-requests
is enabled for that page.

The ssi.conditional-requests directive can be enabled or disabled
globally and/or in any context. It is disabled by default.

An index.shtml which only includes deterministic SSI commands such as:
<!--#echo var="LAST_MODIFIED"-->
is a trivial example of a dynamic SSI page that is cacheable.