Merge updated Pool lib from trunk.
Fixes #1252.
Fixes #2696.
Fixes #4960.
Fixes #5526.
Fixes #5568.
Fixes #5700.
Merge link fixes for pool and timer.
Merge changes from trunk:

Fix PDF install rule so that it's explicit and automatically invokes a PDF build when specified on the command line.
So "bjam pdfinstall" will now build and install the PDF to the current directory.
This works around some problems that the previous versions had if the user did not have an FO processor installed (basically Daniel James was unable to build the HTML docs for the distribution if the pdfinstall rule was implicit).
Merge pool image fixes for case sensitive filesystems.