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'.

docs/redo-ifchange.md

@@ -0,0 +1,92 @@
+# NAME
+
+redo-ifchange - rebuild target files when source files have changed
+
+# SYNOPSIS
+
+redo-ifchange [targets...]
+
+
+# DESCRIPTION
+
+Normally redo-ifchange is run from a .do file that has been
+executed by `redo`(1).  See `redo`(1) for more details.
+
+redo-ifchange doesn't take any command line options other
+than a list of *targets*.  To provide command line options,
+you need to run `redo` instead.
+
+redo-ifchange performs the following steps:
+
+- it creates a dependency on the given *targets*.  If any
+  of those targets change in the future, the current target
+  (the one calling redo-ifchange) will marked as needing to
+  be rebuilt.
+  
+- for any *target* that is out of date, it calls the
+  equivalent of `redo target`.
+
+- for any *target* that is locked (because some other
+  instance of `redo` or `redo-ifchange` is already building
+  it), it waits until the lock is released.
+  
+redo-ifchange returns only after all the given
+*targets* are known to be up to date.
+
+
+# TIP 1
+
+You don't have to run redo-ifchange *before* generating
+your target; you can generate your target first, then
+declare its dependencies.  For example, as part of
+compiling a .c file, gcc learns the list
+of .h files it depends on. You can pass this information
+along to redo-ifchange, so if any of those headers are
+changed or deleted, your .c file will be rebuilt:
+
+        redo-ifchange $2.c
+        gcc -o $3 -c $2.c \
+            -MMD -MF $2.deps
+        read DEPS <$2.deps
+        redo-ifchange ${DEPS#*:}
+
+This is much less confusing than the equivalent
+autodependency mechanism in `make`(1), because make
+requires that you declare all your dependencies before
+running the target build commands.
+
+
+# TIP 2
+
+Try to list as many dependencies as possible in a single
+call to redo-ifchange.  Every time you run redo-ifchange,
+the shell has to fork+exec it, which takes time.  Plus redo
+can only parallelize your build if you give it multiple
+targets to build at once.  It's fine to have a couple of
+separate redo-ifchange invocations for a particular target
+when necessary (as in TIP 1 above), but try to keep it to a
+minimum.  For example here's a trick for generating a list
+of targets, but redo-ifchanging them all at once:
+
+	for d in *.c; do
+		echo ${d%.c}.o
+	done |
+	xargs redo-ifchange
+
+
+# REDO
+
+Part of the `redo`(1) suite.
+    
+# CREDITS
+
+The original concept for `redo` was created by D. J.
+Bernstein and documented on his web site
+(http://cr.yp.to/redo.html).  This independent implementation
+was created by Avery Pennarun and you can find its source
+code at http://github.com/apenwarr/redo.
+
+
+# SEE ALSO
+
+`redo`(1), `redo-ifcreate`(1), `redo-always`(1), `redo-stamp`(1)