Log any error that occurs calling contentsOfDirectoryAtPath in
unsortedLogFilePaths.
Add DDFILELOGGER_USE_SIMULATED_XATTRS preprocessor variable, and use that
to decide whether to a file extension instead of an xattr to indicate
archival status.

This changes nothing in the build as-is, but it allows us to enable either
codepath for debugging.
Log the actual path and name that we use with setxattr / removexattr, i.e.
the char* not the NSString.