This unifies low-level filesystem walker code for `ls` and `cp`.
New features:
* `ls` now reports relative filenames
* `ls` now prints symlink destination for symlinks
* `cp` now properly always reports errors from the API
* `cp` now reports all the errors back to the client
Example for `ls`:
```
osctl-linux-amd64 --talosconfig talosconfig ls -l /var
MODE SIZE(B) LASTMOD NAME
drwxr-xr-x 4096 Jun 26 2019 .
Lrwxrwxrwx 4 Jun 25 2019 etc -> /etc
drwxr-xr-x 4096 Jun 26 2019 lib
drwxr-xr-x 4096 Jun 21 2019 libexec
drwxr-xr-x 4096 Jun 26 2019 log
drwxr-xr-x 4096 Jun 21 2019 mail
drwxr-xr-x 4096 Jun 26 2019 opt
Lrwxrwxrwx 6 Jun 21 2019 run -> ../run
drwxr-xr-x 4096 Jun 21 2019 spool
dtrwxrwxrwx 4096 Jun 21 2019 tmp
-rw------- 14979 Jun 26 2019 userdata.yaml
```
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
Actual API is implemented in the `init`, as it has access to root
filesystem. `osd` proxies API back to `init` with some tricks to support
grpc streaming.
Given some absolute path, `init` produces and streams back .tar.gz
archive with filesystem contents.
`osctl cp` works in two modes. First mode streams data to stdout, so
that we can do e.g.: `osctl cp /etc - | tar tz`. Second mode extracts
archive to specified location, dropping ownership info and adjusting
permissions a bit. Timestamps are not preserved.
If full dump with owner/permisisons is required, it's better to stream
data to `tar xz`, for quick and dirty look into filesystem contents
under unprivileged user it's easier to use in-place extraction.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
