Split directory recursion stuff from cmd-index.py into drecurse.py.

Split directory recursion stuff from cmd-index.py into drecurse.py.

Also add a new command, 'bup drecurse', which just recurses through a
directory tree and prints all the filenames.  This is useful for timing
performance vs. the native 'find' command.

The result is a bit embarrassing; for my home directory of about 188000
files, drecurse is about 10x slower:

$ time bup drecurse -q ~
real	0m2.935s
user	0m2.312s
sys	0m0.580s

$ time find ~ -printf ''
real	0m0.385s
user	0m0.096s
sys	0m0.284s

time find ~ -printf '%s\n' >/dev/null
real	0m0.662s
user	0m0.208s
sys	0m0.456s