add darwin/osxpb support to cvs

this patch and tar archive will add support for the darwin/osxpb to the current cvs tree.

a couple things to note:

- unpack the tar archive in pgsql/

- the config.guess and config.sub files have been updated by apple to support their new os. i don't think these changes
havebeen folded back in to the main archive yet (at least they aren't in the pgsql cvs yet). these need to be copied
overfrom /usr/libexec/config.* in order to obtain the correct os (e.g. powerpc-apple-darwin1.2 on PB). 

- the diff only patches configure.in so autoconf needs to be rerun

- the situation with darwin's implementation of sysv semaphores is in progress at the moment (the shm/ipc support *is*
thereand seems to work fine). so this patch uses HAVE_SYS_SEM_H to conditionally build the src/backend/port/darwin
semaphorecode (borrowed from qnx4). I've followed the BeOS example of including the necessary sem.h declarations in
src/include/port/darwin.h.this is rather messy at the moment and can be dumped once apple releases a version of PB with
sysvsem built into the kernel. 

- i'm a bit confused over the __powerpc__ tas function in s_lock.c (there i assume for the ppc-linux port). it doesn't
compileat all on darwin so i just added a version that does work on darwin under DARWIN_OS. it's potentially a bit
confusingand s_lock.c should probably be changed to include a better conditional. 

bruce