Make sure we (re)initialize-asdf at some early point: things may have changed after image-dump.

* Use define-command to define and register a command
* Introduce the notion of workspace to store intermediate and target files.
* Update Makefiles for new intended output paths

as I was tired of completion conflicts with driver.lisp.

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.

* extract fasl type from target:
 at least lispworks depends on pathname type and its fasl type varies!
* rucksack not available on clisp (and needed fixes; see our xcvb repo of it)
* accept compile-file failure semantics of clisp
* suppress duplicate entries for a directory in the source-registry:
 The first one wins, with its recurse and exclude flags.
* fix hello and test/runme
* fix various option parsing mistakes

	settings known to xcvb or the individual value of a single one.
Change the default object-directory location to be
	$HOME/.cache/xcvb/common-lisp/<lisp-implementation-version>/
Added format! in driver.lisp to help with debugging and forcing output.
Debugged CMUCL support in driver.
Fixed implementation-identifier support in extract-target-properties.
Allowed Makefile in example-1 to parameterize on lisp implementation.
Refactored how option-specs are specified for command line options.
Removed dead code paths from simple-build backend that dealt with
	"xcvb.mk" and the makefile output file.
Muffled warnings when loading asdf.
Added target-asdf-version setting which you can query with show-settings.
Started DONE.rest

Notably fixed a typo in removing feature*s*, as noticed by Theam Yong Chew. Thanks!

We were using CCL's run-program, that spawned a thread that reset the
pthread_sigmask. Evil. We now use a run-program/process-output-stream
based on iolib and short-circuit the implementation's. Sigh.

transient threads created by posix_spawn. Sigh.

Also, supersede some sbcl contrib asdf systems with appropriate requires.

Fix variable names in farmer.
Tweak Makefile to avoid unnecessary work.

Easy functionality: version, ssr and a2x work.

* separated searched build registry from traversed grain registry.
* started a virtual-pathnames layer
* small farmer progress.

* cleanup previous clisp and ccl tweaks after discussing with maintainers.
* another pass of documentation, particularly after interaction with juanjo.

* workaround CLISP 2.48 bug by deleting old file if present.
* also don't apply-output-pathname-translations on dependency-op output-files

(feature implemented for geoff wozniak).
tests now work with clisp (they didn't???).
some docs.