Fix creation of incremental archives when a subdirectory becomes a mountpoint.

Fix creation of incremental archives when a subdirectory becomes a mountpoint.

When used with --listed-incrfental and --one-file-system, tar
was unable to cope with directories that had once been part of the
dumped system and became separate mount points afterwards.  The bug
has been reported and a fix proposed by Nathan Stratton Treadway.

* src/incremen.c (procdir): If one_file_system_option is in effect,
clear out dumpdir info from the directory.
* tests/Makefile.am: Add new testcase.
* tests/listed05.at: New testcase.
* tests/testsuite.at (AT_PRIVILEGED_PREREQ): New macro.
include listed05.at.
* THANKS: Update.