Handle exit codes when running from the Lisp REPL.
Driver tweaks; notably control conditions when loading an ASDF system.

Notably, pass around the location of the xcvb-program we create in the cache.

This allows the examples to be created without having to install
an XCVB. The primary implementation is in bootstrap.lisp and
additional code in driver.lisp.
+ The created XCVB is made with one of the supported lisp
implementations even though the enclosing lisp invocation might
not be.
+ Refactored lisp-invocation into its own package.
+ The cache-description file contains notes about how the cache
should function. The cache is both the multi-user cache as in
distcc, ccache, etc, but also the user specific workspace for
where to dump fasl files and whatnot.

Updating versions.
Thinking about rehauling the manifest format.

Be more conservative wrt depending on asdf.