Updated TPluginManager. Plugin handlers can now be specified wia macros

in a list of plugin directories. These new features are implemented via
two new methods described below. These changes are fully backward compatible
and plugin descriptions in rootrc files still work. However, using the new
macros we only load in program memory the handlers for the needed plugins,
instead of the almost 100 handlers of all plugins. For example after starting
root.exe, only these handlers are loaded:

root [0] gPluginMgr->Print()
=====================================================================
Base                 Regexp        Class              Plugin
=====================================================================
TSystem              ^rfio:        TRFIOSystem        RFIO
TSystem              ^castor:      TRFIOSystem        RFIO
TSystem              ^dcache:      TDCacheSystem      DCache
TSystem              ^dcap:        TDCacheSystem      DCache
TSystem              ^alien:       TAlienSystem       RAliEn
TSystem              ^root:        TXNetSystem        Netx
=====================================================================
6 plugin handlers registered
[*] plugin not available
=====================================================================

For more see below:


void TPluginManager::LoadHandlersFromPluginDirs(const char *base)

Load plugin handlers specified via macros in a list of plugin
directories. The $ROOTSYS/etc/plugins is the default top plugin directory
specified in $ROOTSYS/etc/system.rootrc. The macros must have names
like <BaseClass>/PX0_<PluginClass>.C, e.g.:
   TFile/P10_TRFIOFile.C, TSQLServer/P20_TMySQLServer.C, etc.
to allow easy sorting and grouping. Macros not beginning with 'P' and
ending with ".C" are ignored. If base is specified only plugin macros for
that base class are loaded. The macros typically should look like:

   void P10_TDCacheFile()
   {
       gPluginMgr->AddHandler("TFile", "^dcache", "TDCacheFile",
          "DCache", "TDCacheFile(const char*,Option_t*,const char*,Int_t)");
   }

In general these macros should not cause side effects, by changing global
ROOT state via, e.g. gSystem calls, etc. However, in specific cases
this might be useful, e.g. adding a library search path, adding a specific
dependency, check on some OS or ROOT capability or downloading
of the plugin.


Int_t TPluginManager::WritePluginMacros(const char *dir, const char *plugin)

Write in the specified directory the plugin macros. If plugin is specified
and if it is a base class all macros for that base will be written. If it
is a plugin class name, only that one macro will be written. If plugin
is 0 all macros are written. Returns -1 if dir does not exist, 0 otherwise.
This method allows the regeneration of the entire tree of plugin macros
in case we ever decide to change something in the macros.


git-svn-id: http://root.cern.ch/svn/root/trunk@19373 27541ba8-7e3a-0410-8455-c3a389f83636