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_whichdo.py

@@ -0,0 +1,34 @@
+import sys, os
+
+import vars_init
+vars_init.init_no_state()
+
+import paths
+from logs import err
+
+
+def main():
+    if len(sys.argv[1:]) != 1:
+        err('%s: exactly one argument expected.\n' % sys.argv[0])
+        sys.exit(1)
+
+    want = sys.argv[1]
+    if not want:
+        err('cannot build the empty target ("").\n')
+        sys.exit(204)
+
+    abswant = os.path.abspath(want)
+    pdf = paths.possible_do_files(abswant)
+    for dodir, dofile, basedir, basename, ext in pdf:
+        dopath = os.path.join('/', dodir, dofile)
+        relpath = os.path.relpath(dopath, '.')
+        exists = os.path.exists(dopath)
+        assert '\n' not in relpath
+        print relpath
+        if exists:
+            sys.exit(0)
+    sys.exit(1)   # no appropriate dofile found
+
+
+if __name__ == '__main__':
+    main()