error message if the installation directory layout is messed up (or at least,
something more useful than the behavior exhibited in bug #4787).  During
postmaster startup, check that get_pkglib_path resolves as a readable
directory; and if ParseTzFile() fails to open the expected timezone
abbreviation file, check the possibility that the directory is missing rather
than just the specified file.  In case of either failure, issue a hint
suggesting that the installation is broken.  These two checks cover the lib/
and share/ trees of a full installation, which should take care of most
scenarios where a sysadmin decides to get cute.