Add DOH Exit() and SetExitHandler()

Exit() is a wrapper for exit() by default, but SetExitHandler() allows
specifying a function to call instead.

This means that failures within DOH (e.g. Malloc() failing due to lack
of memory) will now perform cleanup such as removing output files.

This commit also cleans up exit statuses so SWIG should now reliably
exit with status 0 if the run was successful and status 1 if there was
an error (or a warning and -Werror was in effect).

Previously in some situations SWIG would try to exit with the status set
to the number of errors encountered, but that's problematic - for
example if there were 256 errors this would result in exit status 0 on
most platforms.  Also some error statuses have special meanings e.g.
those defined by <sysexits.h>.

Also SWIG/Javascript tried to exit with status -1 in a few places (which
typically results in exit status 255).

CHANGES.current

@@ -7,6 +7,31 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-03-06: olly
+	    SWIG should now reliably exit with status 0 if the run was
+	    successful and status 1 if there was an error (or a warning and
+	    -Werror was in effect).
+
+	    Previously in some situations SWIG would try to exit with the
+	    status set to the number of errors encountered, but that's
+	    problematic - for example if there were 256 errors this would
+	    result in exit status 0 on most platforms.  Also some error
+	    statuses have special meanings e.g. those defined by <sysexits.h>.
+	    Also SWIG/Javascript tried to exit with status -1 in a few places
+	    (which typically results in exit status 255).
+
+2022-03-03: olly
+	    #1901 #2223 SWIG should now always exit cleanly if memory
+	    allocation fails, including removing any output files created
+	    during the current run.
+
+	    Previously most places in the code didn't check for a NULL return
+	    from malloc()/realloc()/calloc() at all, typically resulting in
+	    undefined behaviour; some places used assert() to check for a NULL
+	    return (which is a misuse of assert() and such checks disappear if
+	    built with NDEBUG defined leaving us back with undefined
+	    behaviour).
+
 2022-03-03: olly
 	    #891 Report errors for typemap attributes without a value
 	    (previously SWIG segfaulted) and for typemap types with a value