Defer validity checks until path gets to the remote side, where they
actually make sense. Add simple checks for download directory path to
ensure it's not relative, since one cannot know what current working
directory of the remote process is.
There're too many functions and types to consider, and benefits of not
using LFS macros aren't that big (I was thinking of using fts(3) but
that may not happen soon or at all).
Additionally,
* always close file descriptor on error in cached_file_open (FD leak),
* only store file descriptor to tr_cached_file on success,
* call ftruncate after xfsctl-based preallocation so that correct size
is reported by the system.
