importd: Always specify file unpacked by tar

Despite popular belief, the default file extracted by GNU tar is not stdin.  It
is the value of the TAPE environment variable, falling back on a compile-time
constant.  On my system, the default value is /dev/full, which causes tar to
just spin forever due to --ignore-zeros.  Always specifying this flag is the
safe thing to do.

  ~$ tar --show-defaults
  --format=gnu -f/dev/full -b20 --quoting-style=escape
  --rmt-command=/usr/sbin/grmt

See also: ``(tar)defaults'', available via Info viewers, and in HTML form at:
https://www.gnu.org/s/tar/manual/html_node/defaults.html

src/import/import-common.c

@@ -45,7 +45,8 @@ int import_fork_tar_x(const char *path, pid_t *ret) {
                        "--ignore-zeros",
                        "--numeric-owner",
                        "-C", path,
-                       "-px",
+                       "-pxf",
+                       "-",
                        "--xattrs",
                        "--xattrs-include=*",
                        use_selinux ? "--selinux" : "--no-selinux",