[PD-dev] updated instance handling and made pull request to libpd

[Miller Puckette]

I've made the necessary changes to libpd, mostly a matter of adding lots
of sys_lock() and sys_unlock() calls, and made a pull request out of it
on github.

In doing that I found several remaining problens with Pd's instance/threading
code which I hopefully have fixed.

I added -DPDINSTANCE to the makefile in libpd/samples/c/pdtest_multi ; for
things to work one has to invoke "make ADDITIONAL_CFLAGS=-DPDINSTANCE"
to build pdlib.so (this is done from the makefile in pdtest_multi but if
libpd is already compiled otherwise it's necessary to clean and remake).

The other test I tried (libpd/samples/c/pdtest) appears still to work
whether or not libpd is compiled with PDINSTANCE defined.

As before comments warmly welcomed :)

cheers
Miller