A lazy way to spin up virtual machines to investigate PostgreSQL portability
problems on various OSes.  Goals:

 * most/all of the open source OSes currently in the PostgreSQL build farm
 * all needed packages installed
 * all needed OS configuration (sysctls etc) done
 * PostgreSQL source tree cloned
 * maximum number of options configured
 * compiled, installed, so ready to run and ccache is prewarmed

These operating systems are currently supported:

alpine314:     only basic options, needs more packages
debian11:      all options, 64 bit and 32 bit
dragonfly6:    all options
freebsd13:     all options
freebsd12:     all options
netbsd8:       all options
netbsd9:       all options
openbsd6:      all options except llvm, gssapi
openindiana:   all options except llvm

TODO:  Install civilised stuff like .vimrc, .screenrc
TODO:  Figure out how to move the number of CPUs and RAM size somewhere better.
TODO:  Put in some reasonable settings for VirtualBox and other providers too
TODO:  Find some more OSes.