FS#1090 ensured DW would never rebuild instructions in the same run by forcing subsequent
instruction requests to use the version cached on the first request.  That introduced problems
when the caching of the instructions failed (FS#1334).  This patch allows subsequent rebuilds
when cache storage failed.

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bool page_exists($id, $rev

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The use of realpath() to clean up relative file names caused some
trouble in certain setups relying on symlinks or having restricitve
file structure setups.

This patch replaces all realpath() calls with a PHP only replacement
which should solve those problems.

darcs-hash:20070930184250-7ad00-512ff04c95f57fc9eaf104f80372237a3c94286f.gz

rss syntax extended to include a refresh parameter
 <digits><period>  period can be d,h,m for days, hours, minutes respectively
                   if not specified will default to 4 hours
                   dokuwiki imposes a minimum of 10 minutes

metadata now used "date valid age" (seconds) rather than "date valid end"

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Cache
- add dependency for metadata renderer file
- check metadata for end of page life, "date valid end".

Metadata Renderer
- RSS syntax mode now sets rendered page expiry, "date valid end"
  and includes the feed URL in "relation haspart".

Purgefile
For all wiki installations the purgefile records the earliest
time before which no cache purge (based on data consistency)
is required. Cache files older than this time MAY need to be
purged.
- remove purgeonadd configuration setting

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Reduce overhead by only storing cache hit stats while
the allowdebug option is enabled.

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- add code to ensure page metadata is refreshed along with cache
- bug fix for missing global $conf

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this update changes the processing sequence slightly
to make the cache item's dependencies ($this->depends)
available before _useCache is called.

ie. the dependencies will be available to and modifiable
    by handlers of the PARSER_USE_CACHE event.

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This patch primarily updates p_cached_xhtml() and p_cached_instructions() to
allow their caching logic to be surrounded by an event trigger.

p_cached_xhtml() has been rewritten as the more general p_cached_output() to
support other render output formats besides 'xhtml'. All calls to
p_cached_xhtml() have been changed to refer to the new function.

New event:

name:        PARSER_CACHE_USE
data:        cache object (see below)
action:      determine if cache file can be used
preventable: yes
result:      bool, true to use cache file, false otherwise

Cache operations have been generalised in a new class, cache, extended to
cache_parser, cache_renderer & cache_instructions. Details can be found in
inc/cache.php

For handling of above event, key properties are:
- page, if present the wiki page id,
        may not always be present, e.g. when called for locale xhtml files
- file, source file
- mode, renderer mode (e.g. 'xhtml') or 'i' for instructions

Other changes:
- cache class counts cache hits against attempts, results are stored in
  {cache_dir}/cache_stats.txt
- adds metadata dependency to renderer page cache
- replaces purgefile dependency for renderer cache with metadata
  'relation references' (internal link) dependency for wiki pages only

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