cpdup(1): Some improvements.

cpdup(1): Some improvements.

* Make cpdup retry failed rmdirs after chflags. It already does this
  for remove().

* When deciding whether to copy a file, cpdup should ignore the
  UF_ARCHIVE file flag. If that flag is supported by the destination
  file system but it's cleared on a source file, then multiple
  invocations of cpdup would all copy the source file because its
  flags wouldn't match. OTOH, if the destination filesystem doesn't
  support UF_ARCHIVE, then there's no point in cpdup setting it.

Submitted-by:  Will Andrews <will@firepipe.net>
Dragonfly-bug: https://bugs.dragonflybsd.org/issues/2987
               https://bugs.dragonflybsd.org/issues/2988
               https://bugs.dragonflybsd.org/issues/3067