New script for deferred warnings.

Don't key upgrade attempt on source-registry, since things can also change
in the central-registry, etc.
Also rename required-files to direct-dependency-files
Improve the new convenience function pattern to work with operation initargs.
Implement getcwd on all platforms.
Actually catch sb-grovel messages.

Tests: make u l=ecl instead of make test-upgrade lisp=ecl
Tests: put the setup around the script, not inside;
Tests: start using globals, much easier for debugging

Also, have convenience methods for operation and component designators.
Enhance the incidental traversal functions.
Move compile-file error handling to lisp-build.

Passes tests on CCL and SBCL.

A system named foo/bar will be looked up in a registered file foo.asd.
This makes it possible to sensibly name several systems in a .asd file
and still have asdf find them all by name.
We now use it to not have the asdf header be part of asdf-driver.

A new generic operation build-op makes it possible to deal with
the fact that the default operation for systems need not be load-op,
yet that the end-user does not want to have to know the exact operation
for each of the system he uses.

ensure-pathname has been improved again, and its clients tweaked.

remove-keys was made more sensible by using EQL rather than STRING-EQUAL.

:version need not take the first form in a file, but can take
a path to a subform. This makes it possible to get the exact subform
in a Lisp source file that has the string.
See SUB-OBJECT for how these paths work.

Cleanup of the semantics of ENSURE-FUNCTION on CONSes: use APPLY, not EVAL.

Package cleanup: every file in asdf itself now :use's asdf/driver asdf/upgrade.
Test script fix:
Allegro, like ECL *really* hates that we close *standard-input*,
so stop trying at all.
Tweak some utilities and debug utilities.

This notably includes the former xcvb-driver:run-program/,
a much better replacement for run-shell-command.
This also includes image initialization, dumping, quitting.

TODO: Create a define-package that can make it upgrade itself.