feat: supports resumable uploads

[sigoden]

HTTP PATCH support https://sabre.io/dav/http-patch/

upload_offset=$(curl -I -s http://127.0.0.1:5000/file | tr -d '\r' | sed -n 's/content-length: //p')
dd skip=$upload_offset if=file status=none ibs=1 | \
  curl -X PATCH -H "X-Update-Range: append" --data-binary @- http://127.0.0.1:5000/file

close #335

[sigoden]

webui:

dufs-retry.mp4