don't know what i was smoking when I wrote log_request() like that...

we already check the validity of the format string, but still avoid a gratious fatal() at runtime.

get rid of check_path(), it's overly complicated. Instead, inline open_file() in client_read() and rework open_dir() to just use openat() instead of the complicate dance it was doing. Simplify open_dir() too in the process: if the directory entry for the index is not a regular file, pretend it doesn't exist.