Implement the --sync flag for df. (fixes #3564)
Implement the --sync flag for df. (fixes #3564)

This PR reuses the defined --sync flag for df, which was previously a
no-op, assigns its default value and uses the fact that
get_all_filesystems takes CLI options as a parameter to perform a sync
operation before any fs call.

Fixes #3564

Signed-off-by: anastygnome <0b005a7ea50e8b4c2a938f9884204d19a098e95f@protonmail.com>

uutils/coreutils,uutils/coreutils,uutils/coreutils