SYSV IPC implementation of fast share modes.

SYSV IPC implementation of fast share modes.

It will try sysv IPC first, then if that fails it will try mmap(),
then after that it will try share files.

I have defined USE_SYSV_IPC for Linux, Solaris and HPUX at the
moment. Probably a lot more could have it defined. In fact, the vast
majority of systems support it. Need autoconf again :-)

It should actually be faster than the mmap() version, and doesn't need
any lock files. This means the problem of the share mem file being on
a NFS drive will be gone.