Rename INSTALL_PREFIX to DESTDIR, remove option --install_prefix

INSTALL_PREFIX is a confusing name, as there's also --prefix.
Instead, tag along with the rest of the open source world and adopt
the Makefile variable DESTDIR to designate the desired staging
directory.

The Configure option --install_prefix is removed, the only way to
designate a staging directory is with the Makefile variable (this is
also implemented for VMS' descrip.mms et al).

Reviewed-by: Rich Salz <rsalz@openssl.org>