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/helpers.py

@@ -0,0 +1,26 @@
+import os, errno, fcntl
+
+
+def join(between, l):
+    return between.join(l)
+
+
+def unlink(f):
+    """Delete a file at path 'f' if it currently exists.
+
+    Unlike os.unlink(), does not throw an exception if the file didn't already
+    exist.
+    """
+    try:
+        os.unlink(f)
+    except OSError, e:
+        if e.errno == errno.ENOENT:
+            pass  # it doesn't exist, that's what you asked for
+
+
+def close_on_exec(fd, yes):
+    fl = fcntl.fcntl(fd, fcntl.F_GETFD)
+    fl &= ~fcntl.FD_CLOEXEC
+    if yes:
+        fl |= fcntl.FD_CLOEXEC
+    fcntl.fcntl(fd, fcntl.F_SETFD, fl)