[mod_ssi] basic recursive SSI include virtual (fixes #536)

[mod_ssi] basic recursive SSI include virtual (fixes #536)

EXPERIMENTAL: basic recursive SSI <!--#include virtual="..." -->
Marked experimental since behavior may change in future.

Prior behavior was simpler and treated them all as files included as-is.

New behavior treats all #include virtual="..." targets as SSI files.

In the future, this may change to be a full recursive subrequest and the
virtual path may be treated as a new subrequest and might be something
other than SSI (e.g. might be CGI).  This has not been implemented.

Current behavior processes <!--#include virtual="..." --> as static file
Enable new behavior by setting ssi.recursion-max to value other than 0.
ssi.recursion-max = X to set maximum recusion depth

x-ref:
  "add recursion to the SSI #include directive"
  https://redmine.lighttpd.net/issues/536