Search some standard locations for PEAR library code if DB.php is

Search some standard locations for PEAR library code if DB.php is
not found in PHP's include_path.

Added some workarounds for a bug in the PEAR DB code which ships
with PHP 4.0.6.

(I have now tested the MySQL backend with PEAR code from PHP 4.0.4pl1,
PHP 4.0.5 and PHP 4.0.6.  Only the 4.0.6 code is buggy.   With the new
fixes, all three versions seem to work fine.  The bug is reported
to be fixed in the CVS version of the PHP source code.)

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@568 96ab9672-09ca-45d6-a79d-3d69d39ca109