rm: add --preserve-root=all to protect mounts

rm: add --preserve-root=all to protect mounts

* src/remove.c (rm_fts): With the --preserve-root=all extension,
reject command line arguments that are mount points.
* src/remove.h (rm_options): Add preserve_all_root to store config.
* src/mv.c (rm_option_init): Init preserve_all_root to false.
* src/rm.c (main): Init preserve_all_root as per option.
(usage): Describe the new option.
* src/remove.c (rm_fts): Lookup the parent device id,
and reject the cli argument if a separate file system.
* tests/rm/one-file-system.sh: Add a test case.
* NEWS: Mention the new feature.