Use `#pragma GCC poison` (supported since GCC 3, maybe earlier) when
compiling with GCC to help prevent direct uses being introduced for
functions which DOH provides a wrapper for.
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).
Previously code in the SWIG tool didn't handle allocation failures
well. Most places didn't check for NULL return from
malloc()/realloc()/calloc() at all, typically resulting in undefined
behaviour, and 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).
All C allocations are now done via wrapper functions (Malloc(),
Realloc() and Calloc()) which emit and error and exit with non-zero
status on failure, so a non-NULL return can be relied upon.
Fixes#1901.
