tmpfile-util: add new LINK_TMPFILE_SYNC flag for syncing properly before/after linking in the file

This syncs the data before linking it in, and both data + dir once done.
This should give us proper semantics for installing files safely into
the fs.

src/basic/tmpfile-util.h

@@ -32,6 +32,7 @@ int fopen_tmpfile_linkable(const char *target, int flags, char **ret_path, FILE
 
 typedef enum LinkTmpfileFlags {
         LINK_TMPFILE_REPLACE = 1 << 0,
+        LINK_TMPFILE_SYNC    = 1 << 1,
 } LinkTmpfileFlags;
 
 int link_tmpfile_at(int fd, int dir_fd, const char *path, const char *target, LinkTmpfileFlags flags);