fixing O(too_much) bug in recursive list_subdirs

[biappi]

for some reason this function was scanning subdirectories more than once, resulting in a really sub-ottimal performance.

with this patch, i was able to reduce build of the project i'm working on from:
real 1m1.629s

to:
real 0m1.923s

on some occasion i was also able to fix an:
(ino-env)Minene:ino willy$ time ino build > /dev/null
make: /bin/sh: Argument list too long
make: *** [.build/uno/src/iec_driver.d] Error 1

real 0m32.280s
user 0m16.205s
sys 0m13.660s

error.