Directory reorg: move code into redo/, generate binaries in bin/.

It's time to start preparing for a version of redo that doesn't work
unless we build it first (because it will rely on C modules, and
eventually be rewritten in C altogether).

To get rolling, remove the old-style symlinks to the main programs, and
rename those programs from redo-*.py to redo/cmd_*.py.  We'll also move
all library functions into the redo/ dir, which is a more python-style
naming convention.

Previously, install.do was generating wrappers for installing in
/usr/bin, which extend sys.path and then import+run the right file.
This made "installed" redo work quite differently from running redo
inside its source tree.  Instead, let's always generate the wrappers in
bin/, and not make anything executable except those wrappers.

Since we're generating wrappers anyway, let's actually auto-detect the
right version of python for the running system; distros can't seem to
agree on what to call their python2 binaries (sigh). We'll fill in the
right #! shebang lines.  Since we're doing that, we can stop using
/usr/bin/env, which will a) make things slightly faster, and b) let us
use "python -S", which tells python not to load a bunch of extra crap
we're not using, thus improving startup times.

Annoyingly, we now have to build redo using minimal/do, then run the
tests using bin/redo.  To make this less annoying, we add a toplevel
./do script that knows the right steps, and a Makefile (whee!) for
people who are used to typing 'make' and 'make test' and 'make clean'.

redo/cmd_ifchange.py

@@ -0,0 +1,65 @@
+import os, sys, traceback
+
+import vars_init
+vars_init.init(sys.argv[1:])
+
+import vars, state, builder, jwack, deps
+from logs import debug2, err
+
+def should_build(t):
+    f = state.File(name=t)
+    if f.is_failed():
+        raise builder.ImmediateReturn(32)
+    dirty = deps.isdirty(f, depth='', max_changed=vars.RUNID,
+                         already_checked=[])
+    return f.is_generated, dirty == [f] and deps.DIRTY or dirty
+
+
+def main():
+    rv = 202
+    try:
+        if vars_init.is_toplevel and vars.LOG:
+            builder.close_stdin()
+            builder.start_stdin_log_reader(
+                status=True, details=True,
+                pretty=True, color=True, debug_locks=False, debug_pids=False)
+        if vars.TARGET and not vars.UNLOCKED:
+            me = os.path.join(vars.STARTDIR,
+                              os.path.join(vars.PWD, vars.TARGET))
+            f = state.File(name=me)
+            debug2('TARGET: %r %r %r\n'
+                   % (vars.STARTDIR, vars.PWD, vars.TARGET))
+        else:
+            f = me = None
+            debug2('redo-ifchange: not adding depends.\n')
+        jwack.setup(1)
+        try:
+            targets = sys.argv[1:]
+            if f:
+                for t in targets:
+                    f.add_dep('m', t)
+                f.save()
+                state.commit()
+            rv = builder.main(targets, should_build)
+        finally:
+            try:
+                state.rollback()
+            finally:
+                try:
+                    jwack.force_return_tokens()
+                except Exception, e:  # pylint: disable=broad-except
+                    traceback.print_exc(100, sys.stderr)
+                    err('unexpected error: %r\n' % e)
+                    rv = 1
+    except KeyboardInterrupt:
+        if vars_init.is_toplevel:
+            builder.await_log_reader()
+        sys.exit(200)
+    state.commit()
+    if vars_init.is_toplevel:
+        builder.await_log_reader()
+    sys.exit(rv)
+
+
+if __name__ == '__main__':
+    main()