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_ifcreate.py

@@ -0,0 +1,26 @@
+import sys, os
+import vars, state
+from logs import err
+
+
+def main():
+    try:
+        me = os.path.join(vars.STARTDIR,
+                          os.path.join(vars.PWD, vars.TARGET))
+        f = state.File(name=me)
+        for t in sys.argv[1:]:
+            if not t:
+                err('cannot build the empty target ("").\n')
+                sys.exit(204)
+            if os.path.exists(t):
+                err('redo-ifcreate: error: %r already exists\n' % t)
+                sys.exit(1)
+            else:
+                f.add_dep('c', t)
+        state.commit()
+    except KeyboardInterrupt:
+        sys.exit(200)
+
+
+if __name__ == '__main__':
+    main()