List build pool

My first time using GThreadPools - what a great tool!
This gave modest improvement in list build time, eg reduced from ~90 seconds to ~60 seconds for my /home folder (cache flushed each time by `echo 3 | sudo tee /proc/sys/vm/drop_caches`.
Other changes include some tidy ups and also better reporting of FTS errors.  This requires using error.h/error() so needed to rename the other error() calls to rm_error.