since we don't use setter/getters for the other options it doesn't make
sense to have them for the keep-alive function

As soon as something goes wrong while querying a http server
do not reuse the same connection again, its state is undefined.

In addition, check the connection for feof() before reusing it.

Using a static context for the connection pool allows connection
reuse throughout the whole application without additional changes
in other places.

The DokuHTTPClient is now able to keep connections alive. This
feature is enabled by default. It can be disabled with
    $client->setKeepAlive(false);
and asked with
    $client->isKeepAlive();.

Before this change metaFiles didn't return anything for ids where the
part without the namespace needs (utf-8) filename escaping.

This commit introduces a new define P_GET_FIRST_HEADING_METADATA_LIMIT
that can be set in preload.php in order to change the limit for how many
pages the first heading shall be loaded from metadata in
p_get_first_heading. Changing this is probably most interesting for
Wikis with a lot of pages where loading the title index costs a
significant amount of time and memory.

This makes sure that the indexer is also called for hidden pages so they
aren't missing in the title index and digest subscriptions work for
them, too. Hidden pages are already filtered from the search results.

This commit fixes two bugs that occurred when msg() was called after
html_msgarea() had already been called.
- the $MSG array is now cleared when it has been printed (otherwise $MSG
  has been printed again when another msg() call was done)
- headers_sent() didn't work for me, it always reported false although
  html_msgarea() had already been called which might be explainable with
  output buffering. This makes msg() now depend on the first call of
  html_msgarea() or headers_sent() in order to not to break msg() in
  ajax requests etc.

This change makes p_get_first_heading load the title index when more
than 11 requests that caused a call to p_get_metadata have already been
done. This means that small pages and the breadcrums won't trigger the
loading of the title index but for larger pages or the sitemap the title
index will be used. This is necessary because every call to
p_get_metadata can trigger the parsing and rendering of a whole page and
there can be many calls when useheading is activated and e.g. the
index/sitemap page is displayed.

Additionally this adds a small title cache that caches titles requested
from p_get_metadata.

Further tests should be done how this affects memory usage and how often
the index loading is triggered in order to see if that parameter should
be adjusted.

p_get_metadata has a $render parameter that has been disabled by the
restructuring of metadata rendering. This change reactivates it so
rendering metadata can be prevented. This is e.g. used in the search and
in some plugins like indexmenu that use p_get_first_heading. The default
of the parameter has been changed to true as otherwise the new caching
structure won't work as almost all calls to p_get_metadata don't set the
$render parameter.
The indexer call to p_get_first_heading has been changed to set $render
to true as in the indexer only one page will be rendered and the title
in the index should really be the current one.
This does not fix the problem that rendering pages with lots of links or
displaying the index can cause the parsing/rendering of a lot of pages.

Filenames can't contain ":" on windows and the lock directory contained
the unescaped page id. The lock function tries in an endless loop to
create the lock directory when it fails and the directory doesn't exist.
Just escaping the directory name won't work as then the filename length
limit will be quickly hit when using deep namespace structures with utf8
names. Thus using the md5sum seems to be the best solution. Perhaps the
lock function could also be changed to create a file with that name that
contains the id so the id can be retrieved for debugging purposes.

- better use of Latin terminology for DokuWiki
- ACL is not translated as AAA (Aditus Administrationis Aditus) but
  with ICA (Index Custodiae Aditus), which is a more literal translation
- minor corrections of grammar and logic
- The word "namespace" is more literally translated "spatium
  nominis", but according to other translations (like the Italian one),
  I preferred to use "genus".

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As suggested by Robin Getz locale .txt files can now be duplicated and
changed in the conf/lang/ directory and conf/plugin_lang/$plugin/
directory for plugins.

This avoids disk writes when not needed and possibly also xhtml
rendering when the metadata needs to be rendered but xhtml doesn't
(unless the metadata file is changed).