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'.

Documentation/redo-ifchange.md

@@ -1,92 +0,0 @@
-# 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-- for any *target* that is out of date, it calls the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)