shared/fdset: add detailed debug logging to fdset_new_fill()

shared/fdset: add detailed debug logging to fdset_new_fill()

Currently, when fdset_new_fill() fails to open /proc/self/fd or
encounters an error while processing individual file descriptors
(such as fcntl or fstat failures), it returns a silent error code.

For debugging rarely reproducible failures it becomes difficult to
know the exact cause of failure
This commit updates the function to use log_debug_errno() for all
error paths and hence  provides better visibility into why FD collection
failed, including the path of the problematic FD (via fd_get_path)
and its inode type.