ftpfs: use clearstatcache() before all filesize calls
ftpfs: hide debug messages when not in debug mode
ftpfs: make truncate() work again on opened files
ftpfs: swap size and modify in cache name to allow sort-by-date
ftpfs: skip loading on length=0
ftpfs: add fsc_truncate to allow truncating cached files
ftpfs: fsc_flush rename instead calling fsc_load again

this increases performance as the file will not be loaded pointlessly (and storage, since the -dirty file doesn't get left over)