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

Extend RUN-PROGRAM/PROCESS-OUTPUT-STREAM to accept a NIL processor, inheritting stdout.
Stop exporting private XCVB entry points RUN, DO-RUN, RUN-COMMANDS, RUN-COMMAND.

+ Partial changes to get xcvb to build a binary of xcvb.
+ Extending :create-image to support executables.
+ Support for command line arguments when resuming the executable.
  These are passed to the entry-point. However, each lisp vendor
  mangles the argv a little and it needs work to be really right.
  For now it is good enough.
+ dump-image for ecl got hacked upon, but it is currently broken.
+ Changed example-1 to produce an executable, but currently this
  only works for lisp implementations which can produce them. This
  needs more consideration in the future, for example, the
  "executable" could be a script which invokes an image.
+ simple-build can now accept targets with empty computations.
  This implements the "phony target" idea in simple-build.

since ASDF was running the old uninterned perform method while ironclad was
adding its :around method to the new ASDF perform symbol and generic function
that is not active yet.

Try to be less dumb when deciding whether to use enough-namestring.

TODO: need to better control the cwd vs *default-pathname-defaults*

Converted the makefile style transformation of a computation into
a command into a more generalized external command form of the
computations.  Condensed functionality from driver-commands.lisp and
makefile-backend.lisp into external-commands.lisp. This is because at
this time both the makefile backend and a new run-program back end will
use them.

Added a run-program based backend called "simple-build" which will execute
the commands necessary to build the dependencies in the graph. At this
time it is a serial execution of the dependencies. The functionality is
not yet complete and currently broken as of this checkin.

Fixed up the xcvb.asd file to denote the new dependencies.