Silence more compiler warnings;  fix an instance of potential undefined behaviour from signed overflow.

build (all other definition where correct).

compilition with libedit on OSX 10.5

module dictionary has also been cleared.

writeback=True values are written to the backing store when assigned to
the shelf.  Add test to confirm that this happens.  Doc patch and added
test by Robert Lehmann.  I also fixed the cross references to the sync
and close methods.

close method refers to an identifier in the global module namespace.
This means that when __del__ is called during interpreter shutdown (if,
for example, the calling program still has a pointer to the shelf),
sometimes that global identifier would wind up being None, causing
mysterious 'ignored' exceptions.  This patch checks for the possible None
value first before using the global, thus avoiding the error messages.

#7712: add a temp_cwd context manager to test_support and use it in regrtest to run all the tests in a temporary directory, saving the original CWD in test_support.SAVEDCWD. Thanks to Florent Xicluna who helped with the patch.

in XML processing instructions and comments.  These raw characters are
allowed by the XML specification, and are necessary when outputting e.g.
PHP code in a processing instruction.  Patch by Neil Muller.